Light weight proppant with improved strength and methods of making same

ABSTRACT

Methods are described to make strong, tough, and lightweight whisker-reinforced glass-ceramic composites through a self-toughening structure generated by viscous reaction sintering of a complex mixture of oxides. The invention further relates to strong, tough, and lightweight glass-ceramic composites that can be used as proppants and for other uses.

This application is a continuation of U.S. patent application Ser. No.13/846,232, filed on Mar. 18, 2013, which, in turn, is a continuation ofInternational Patent Application No. PCT/US2011/051712, filed on Sep.15, 2011, which, in turn, claims the benefit under 35 U.S.C. §119(e) ofprior U.S. Provisional Patent Application No. 61/384,875, filed Sep. 21,2010, the disclosures of which are incorporated in their entirety byreference herein.

BACKGROUND OF THE INVENTION

The present invention relates to methods to make strong, tough, andlightweight whisker-reinforced glass-ceramic composites. The method caninvolve forming a self-toughening structure generated by viscousreaction sintering of a complex mixture of oxides. Thewhisker-reinforced glass-ceramic preferably is strong, tough, and/orlightweight. The present invention further relates to strong, tough, andlightweight glass-ceramic composites used as proppants and for otheruses including, but not limited to, armor plating, electronic, optical,high-temperature structural materials and applications, as a lowdielectric constant substrate material in high-performance packagingapplications; or window materials for the mid-infrared range.

The use of certain inorganic whiskers and fibers to reinforce glasses,glass ceramics, and ceramics has been known and practiced. Whiskers aretypically characterized as relatively short, single-crystal fibers ofsmall diameter, typically less than 100 microns. Fibers on the otherhand can be multicrystalline or amorphous and are long enough to be usedin woven or other types of interlocking networks, filter tows or fabric.Whiskers are typically incorporated in a selected glass or ceramicmatrix as a randomly dispersed phase.

Fibers are more commonly used in an oriented or interlocking alignment.Load transfer by the matrix to the fibers through shear is the means bywhich fibers strengthen glass or ceramic bodies. The load transfersstress from the glass or ceramic matrix to the relatively long and highmodulus fibers. The fibers can also impede crack initiation andpropagation through the matrix material.

Whiskers can provide strengthening in a similar manner, but loadtransfer to the whiskers by the matrix is more limited because of thelimited length and aspect ratio of the whiskers. Because whiskers arerelatively short, they cannot carry as much load compared to the longerfibers. It is more difficult to take full advantage of the intrinsicstrength of whiskers compared to fibers for this reason. Whiskerreinforcement in ceramic and glass-ceramic materials is often used toincrease toughness. A toughened ceramic material improves crackresistance, increases fatigue lifetime and/or provides a noncatastrophicmode of failure. Noncatastrophic failure is highly desirable inapplications where repair can be facilitated and information aboutfailure conditions is important.

Silicon carbide, silicon nitride, alumina, and carbon whiskers have allbeen used to reinforce non-metallic matrices. For example, U.S. Pat. No.4,324,843 describes SiC fiber reinforced glass-ceramic composite bodieswhere the glass-ceramic matrix is an aluminosilicate composition. U.S.Pat. No. 4,464,192 describes whisker-reinforced glass-ceramic compositesof an aluminosilicate composition. U.S. Pat. No. 4,464,475 describessimilarly reinforced glass-ceramics with barium osumilite as thepredominant crystal phase.

The use of whiskers in ceramic composites can improve the fracturetoughness of the ceramic composite because of the whiskers' ability toabsorb cracking energy. The whiskers appear to toughen the composites bydeflecting crack propagation, bridging cracks and by whisker “pull-out.”Whisker “pull-out” occurs when the ceramic matrix at the whisker-matrixinterface cracks. When a crack-front propagates into the composite, manyof the whiskers can bridge the crack line and extend into the ceramicmatrix surrounding the crack. For the crack to grow or propagate throughthe ceramic, these whiskers must be either broken or pulled out of thematrix. As these whiskers are pulled out of the matrix, they provide abridging force across the faces of the crack, reducing the intensity ofthe stress at the crack tip. In this way, the whiskers absorb the energythat would propagate the crack. Whisker pull-out reduces the tendency ofa composite to crack and also inhibits crack propagation. U.S. Pat. Nos.4,543,345; 4,569,886; and 4,657,877 relate to silicon carbidewhisker-reinforced ceramic composites.

Perlite (sometimes spelled pearlite) is an amorphous volcanic glass thathas a relatively high water content. Perlite is formed by the hydrationof obsidian, a naturally occurring volcanic glass formed as an extrusiveigneous rock. The typical chemical composition of perlite is SiO₂:69-72%, Al₂O₃: 12-18%, K₂O: 3-4.5%, Na₂O: 3-4.5%, CaO: 0.1-0.2%, MgO:0.2-0.5%, Moisture: 2-4%, where all percentages are weight percent.Perlite has the unusual property of greatly expanding when sufficientlyheated. When perlite reaches temperatures of 850-900° C., it softens(since it is a glass). Water trapped in the structure of the perlitevaporises and escapes, causing an expansion of the perlite to 7-16 timesits original volume. The expansion process of perlite requires rapidheating (around 900° C./min) and then removal of the particle from theheat zone. The expansion creates countless tiny bubbles leading to verylow density. Special heating approaches such as steam or flame heatingare usually employed to achieve the required heating rate. In a typicalindustrial furnace with a heating rate less than 200° C./min. the rawperlite cannot be expanded. Since perlite is a form of natural glass, itis chemically inert and has a pH around 7. Unexpanded perlite has a bulkdensity around 1.1 g/cm³ and expanded perlite has a bulk density ofabout 30-150 kg/m³. Expanded perlite is used in a variety of industrialapplications as a filler because of its ability to expand and fill voidspaces and because of its relatively low specific gravity. The majorityof the applications of perlite are in building construction in theexpanded form due to its low density, low thermal and acousticalconductivity, and non-flammability. Perlite is used as a loose fillinsulation in masonry construction, an aggregate in concrete, anaggregate in Portland cement and an aggregate in gypsum plasters.Perlite is a relatively low cost material compared to other materialsused in the formulation of glass-ceramic composites. The cost of perliteis approximately the same as sand. Perlite has been used in proppantsand other ceramics primarily for its relatively low specific gravity.U.S. Patent Application Nos. 2005/0096207, 2006/0162929 and2006/0016598, and U.S. Pat. No. 7,160,844 describe the use of perlite asa filler in proppants. U.S. Patent Application Nos. 2006/0177661 and2009/0038797 describe the use of perlite as a lightweight template inproppants.

The production of glass-ceramic composites with whisker or fiberreinforcement usually involves dispersion of the whiskers or fibers in agreen body prior to firing or sintering the green body to produce thefinal glass-ceramic reinforced composite. The methods in U.S. Pat. Nos.4,543,345; 4,569,886; and 4,657,877 recite preformed whiskers dispersedin a ceramic precursor prior to forming a green body for sintering.Processes involving dispersion of preformed whiskers in a green bodymaterial have been difficult to successfully implement because whiskershave a tendency to agglomerate resulting in non-uniform concentrationsof whiskers throughout the green body and ultimately in the ceramiccomposite. Non-uniform whisker concentration results in significantvariance in the extent of reinforcement and toughening. As the percentby weight of whiskers in a green body material increases, agglomerationand clumping of whiskers increases. In addition, powdered ceramicprecursor material may become imbedded within clumped whiskers. Aftersintering, the presence of these powders can significantly weaken thewhiskers' reinforcing abilities.

A variety of granular particles are widely used as propping agents tomaintain permeability in oil and gas formations. Three grades ofproppants are typically employed: sand, resin-coated sand and ceramicproppants. Conventional proppants exhibit exceptional crush strength butalso extreme density. A typical density of ceramic proppants exceeds 100pounds per cubic foot. Proppants are materials pumped into oil or gaswells at extreme pressure in a carrier solution (typically brine) duringthe hydrofracturing process. Once the pumping-induced pressure isremoved, proppants “prop” open fractures in the rock formation and thuspreclude the fracture from closing. As a result, the amount of formationsurface area exposed to the well bore is increased, enhancing recoveryrates. Proppants also add mechanical strength to the formation and thushelp maintain flow rates over time. Proppants are principally used ingas wells, but do find applications in oil wells.

Relevant quality parameters include: particle density (low density isdesirable), crush strength and hardness, particle size (value depends onformation type), particle size distribution (tight distributions aredesirable), particle shape (spherical shape is desired), pore size(value depends on formation type and particle size, generally smaller isbetter), pore size distribution (tight distributions are desirable),surface smoothness, corrosion resistance, temperature stability, andhydrophilicity (hydro-neutral to phobic is desired). Lighter specificgravity proppants can be desirable, which are easier to transport in thefracturing fluid and therefore can be carried farther into the fracturebefore settling out and which can yield a wider propped fracture thanhigher specific gravity proppants.

Proppants used in the oil and gas industry are often sand and man-madeceramics. Sand is low cost and light weight, but low strength; man-madeceramics, mainly bauxite-based ceramics or mullite based ceramics aremuch stronger than sand, but heavier. Ceramic proppants dominate sandand resin-coated sand on the critical dimensions of crush strength andhardness. They offer some benefit in terms of maximum achievableparticle size, corrosion and temperature capability. Extensivetheoretical modeling and practical case experience suggest thatconventional ceramic proppants offer compelling benefits relative tosand or resin-coated sand for most formations. Ceramic-driven flow rateand recovery improvements of 20% or more relative to conventional sandsolutions are not uncommon.

Ceramic proppants were initially developed for use in deep wells (e.g.,those deeper than 7,500 feet) where sand's crush strength is inadequate.In an attempt to expand their addressable market, ceramic proppantmanufacturers have introduced products focused on wells of intermediatedepth.

Resin-coated sands offer a number of advantages relative to conventionalsand. First, resin coated sand exhibits higher crush strength thanuncoated sand given that resin-coating disperses load stresses over awider area. Second, resin-coated sands are “tacky” and thus exhibitreduced “proppant flow-back” relative to conventional sand proppants(e.g. the proppant stays in the formation better). Third, resin coatingtypically increases sphericity and roundness thereby reducing flowresistance through the proppant pack.

Ceramics are typically employed in wells of intermediate to deep depth.Shallow wells typically employ sand or no proppant.

SUMMARY OF THE INVENTION

A feature of the present invention is to provide a composite having awhisker phase and an amorphous phase.

A feature of the present invention is to provide a glass-ceramiccomposite having a mullite whisker phase and an amorphous phase.

A further feature of the present invention is to provide a glass-ceramiccomposite having a mullite whisker phase and an amorphous phase in whichthe whiskers are present in a three dimensional non-woven structure.

A further feature of the present invention is to provide a glass-ceramiccomposite having a mullite whisker phase and an amorphous phase in whichthe whiskers are uniformly dispersed.

A further feature of the present invention is to provide a glass ceramiccomposite with a uniform dispersion of relatively small pores withsubstantially spherical shape and a smooth interior pore surface.

A further feature of the present invention is to provide a method formaking a composite having a whisker phase and an amorphous phase,wherein the whisker phase is pre-formed and/or formed in-situ.

A further feature of the present invention is to provide a method formaking a glass-ceramic composite having a mullite whisker phase and anamorphous phase in which the mullite whiskers are pre-formed and/orformed in situ.

A further feature of the present invention is to provide a method formaking strong, tough, and lightweight glass-ceramic matrix compositesthrough a self-toughening structure generated by viscous reactionsintering of a complex mixture of oxides.

A further feature of the present invention is to provide a method formaking strong, tough, and/or lightweight glass-ceramic matrix compositesthrough a self-toughening structure that can be generated by producing auniform dispersion of relatively small pores with substantiallyspherical shape and a smooth interior pore surface, and this can be doneby liberating entrapped water and/or other vaporizable materials from anamorphous material, for instance, during a controlled viscous reactionsintering of a complex mixture of oxides.

A further feature of the present invention is to provide a method formaking strong, tough, and/or lightweight glass-ceramic matrix compositesincluding an optional fluxing agent.

A further feature of the present invention is to provide a glass-ceramiccomposite, such as in the form of a proppant, with superior crushstrength.

A further feature of the present invention is to provide a proppanthaving a superior balance of crush strength and/or buoyancy as shown byspecific gravity.

A further feature of the present invention is to provide a glass-ceramiccomposite, such as in the form of a proppant, with superior resistanceto chemical attack, for instance, from acids and/or aqueous saltsolutions.

A further feature of the present invention is to provide a glass-ceramiccomposite, such as in the form of a proppant, with a smooth externalsurface and containing pores with a high degree of sphericity and asmooth surface on the interior surface of the pores.

A further feature of the present invention is to provide a glass-ceramiccomposite, such as in the form of a proppant, with high strength, lowspecific gravity, and/or high chemical resistance at low cost.

A further feature of the present invention is to provide a proppant thatcan overcome one or more of the disadvantages described above.

To achieve one or more features of the present invention, the presentinvention relates to a method to produce a material, such as acomposite, by forming a green body. The green body can be formed from agreen body material. The green body material can include at least onemetal oxide (such as a first metal oxide and a second metal oxide,wherein the first metal oxide is different from the second metal oxide).The metal oxides are preferably capable of forming whiskers in-situ, forinstance, from or due to reactive or reaction sintering. The in-situwhiskers can be ceramic whiskers, mineral whiskers, metal oxidewhiskers, or any combination thereof. The green body material furtherincludes pre-formed whiskers that can be ceramic pre-formed whiskers,mineral pre-formed whiskers, and/or metal oxide pre-formed whiskers. Thegreen body material further includes at least one whisker promoter. Thegreen body may further include at least one amorphous material thatcontains entrapped water and/or other vaporizable materials. Theamorphous material may have a specific gravity that is lower that theother green body materials thereby contributing to a lower proppantspecific gravity. The method then involves sintering the green bodyunder sintering conditions that preferably are reactive sinteringconditions in order to form a material or sintered body having at leastone whisker phase and at least one amorphous phase. An optional fluxingagent can be provided to enhance mixing of and sintering of the greenbody material. The method can further involve controlling the sinteringprocess to liberate water or other materials entrapped in an amorphousmaterial in a controlled manner such that the liberated gas forms arelatively small number of substantially uniformly distributed poreshaving a substantially spherical shape in the sintered body.

An example of a method for producing a self-toughened high-strengthglass-ceramic composite can be as follows. The method can includeforming a green body from a green body material. The green body materialcan include:

-   -   a) alumina and/or at least one alumina precursor and a siliceous        material in a controlled ratio to form mullite whiskers in a        glass-ceramic composite, and    -   b) a minor amount of mullite whiskers, and    -   c) at least one whisker promoter in the absence of fluorine or        fluorine compounds, and    -   d) perlite and/or at least one amorphous material containing at        least one entrapped vaporizable material such as water, and    -   e) optionally, at least one fluxing agent such as nepheline        syenite, feldspar, clay, and/or similar materials.        The method can include sintering the green body under sintering        conditions to form in situ a glass-ceramic composite with at        least one mullite whisker phase and at least one amorphous        phase. The method can further include controlling the sintering        conditions such that the water in the perlite and/or the        vaporizable material in an amorphous material is liberated to        form a relatively small number of substantially uniformly        distributed pores having substantially spherical shape in the        sintered body.

The present invention further relates to materials, composites, orparticles of the present invention. The material of the presentinvention has a whisker phase and at least one amorphous phase. Thematerial can further include pre-formed whiskers. The in-situ whiskerscan be uniformly distributed throughout the material. The in-situwhiskers can have a concentration that is uniform throughout thematerial. As an option, there is no agglomeration or clumping of thepre-formed and/or in-situ whiskers in the material. The whiskers can bepresent in a three-dimensional non-woven structure or pattern in thematerial. The whisker phase of the pre-formed and/or in-situ whiskerscan be a continuous phase, or can be a non-continuous phase, dependingon the concentration of the whiskers that make the whisker phase. Theamorphous material, such as perlite, can have a lower specific gravitythan the other materials in the green body material. The material of thepresent invention can include a relatively small number of substantiallyuniformly distributed pores having substantially spherical shape in thesintered body.

The present invention further provides a new and improved proppingagent, and a method of making and use thereof, that overcomes theabove-referenced problems and others. The present invention also relatesto a ceramic proppant having a unique microstructure that includeswhiskers arranged in a random alignment, and optionally having reduceddensity, and/or improved strength. The whiskers can be employed toreinforce the ceramic proppant and/or dissipate energy during crackpropagation. The microstructure can also include anisotropic crystals,for example, crystals elongated along the C-axis. The proppant can havea reduced density such that the proppant has a low specific gravitywhile optionally maintaining improved mechanical and/or flexuralstrength. As an option, the present invention provides a proppant havingsubstantially spherical shape and a smooth exterior surface. The presentinvention can include a relatively small number of substantiallyuniformly distributed pores having substantially spherical shape in thesintered body where such pores can contribute to reducing the density ofthe proppant. The small size, uniform distribution, and/or substantiallyspherical shape of the pores contribute to improved toughness, strengthand/or crush resistance of the proppant. The present invention canprovide a proppant having improved resistance to chemical attack, suchas from acids and/or aqueous salt solutions.

The present invention further relates to a method of producing a ceramicproppant that employs a reactive sintering process to form whiskersin-situ through the chemical reaction of raw materials and, optionally,to form a relatively small number of substantially uniformly distributedpores having substantially spherical shape in the ceramic proppant, suchas through the use of controlled sintering of amorphous materials havinginclusions of a vaporizable material. Pores may also be produced throughthe use of pore formers which form pores through chemical reactionand/or thermal decomposition to form a gas. The method allows theporosity, such as pore size, pore size distribution, and/or pore shape,of the proppant, to be controlled. Alterations to the porosity can havea large impact on reducing the specific gravity while maintainingmechanical and/or flexural strength.

The proppant can be used in any application suitable for a proppant. Thepresent invention accordingly relates to a method to prop opensubterranean formation fractions using the proppant.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are intended to provide a further explanation of the presentinvention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this application, illustrate some of the embodiments of thepresent invention and together with the description, serve to explainthe principles of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic showing the structure of a proppant with a hollowtemplate.

FIG. 2 is a SEM image showing the microstructure of in-situ formedmicrowhiskers on the free and fracture surfaces of a proppant.

FIG. 3 is a SEM image showing evidence of pull-out of micro-whiskers ina composite structure.

FIG. 4 is a SEM image showing the texture fracture surface of acomposite after leaching out the glass phase.

FIG. 5 is a chart showing specific gravity and crush strength ofproppants made with varying amounts (in wt %) of alumina, cenospheresand perlite.

FIG. 6 is an X-ray diffraction pattern showing corundum (la-Alumina) andmullite for a composite made with 50 wt % Al₂O₃. 30 wt % Cenosphere, and20 wt % Perlite.

FIG. 7 is a comparison of alumina-cenosphere compositions (CAM) with aselected alumina-cenosphere-perlite composition (all percents are wt %).

FIG. 8 is a SEM image showing the fractured surface of a split-testedpellet with composition: 50 wt % Al₂O₃-30 wt % Cenospheres-20 wt %Perlite.

FIG. 9 is a SEM image showing the fractured surface of a split-testedpellet with composition: 50 wt % Al₂O₃-50 wt % Cenospheres.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to a material (e.g., particles,agglomerates, an article, and the like) that includes whiskers and anamorphous phase. The material can be referred to as a sintered body. Thematerial can be a composite of two or more materials, which, in thiscase, would be at least the whiskers and the amorphous phase present inthe same material. The material can have a smooth and continuoussurface. Optionally, the amorphous phase may contain pores with a smoothinterior surface that are present in the amorphous phase and within thesmooth exterior surface of the material. Optionally, at least onecrystalline particulate phase is present in the material. The materialof the present invention can include whiskers that are in a matrix thatincludes at least one metal oxide, such as silica. The matrix caninclude other components or ingredients as mentioned herein. The matrixis preferably at least amorphous (e.g., having an amorphous phase). Forpurposes of the present invention, the material can include whiskers ora whisker phase and an amorphous phase, for instance, that can includesilica and/or other metal oxides.

For purposes of the present invention, the material of the presentinvention will be described in terms of its preferred form or shape,namely particles that can be used in a variety of end use applications,such as for proppant uses in hydrocarbon recovery. While the preferredshape and preferred materials of the present invention are described indetail below, it is to be understood that this is simply for exemplarypurposes and in no way limits the scope of the present invention withrespect to shape, materials, and/or end uses. While the term “proppant”is used at times in the application, it is understood that this term isnot meant to be limited to its end use application, but for purposes ofthe present invention, it is to be understood that the proppant orparticles, which are used as proppants, can be used in any end useapplication where ceramic material is useful.

Also, for purposes of the present invention, it is to be understood thatthe term “whiskers” as used herein can include what is traditionallyknown as whiskers, namely a length of less than 1 micron, or can includewhat is traditionally know as “fibers,” namely a length of 1 micron ormore, or both.

The material or particles of the present invention can be characterizedas composites and these composites can be glass-ceramic composites dueto the glassy phase or glassy components present in the composite anddue to the ceramic phase or ceramic components present in the composite.

The preferred material or particles of the present invention havewhiskers distributed in a matrix (e.g., glassy matrix), wherein thematrix includes at least one metal oxide, such as silica. The matrix canbe considered amorphous or to have the amorphous phase. Preferably, theamorphous phase is present throughout the material and preferablydistributed in a uniform manner. The whiskers are preferably distributedin a glassy matrix. The glassy matrix can include at least one metaloxide, such as silica or a silicon-containing oxide. The glassy matrixmay contain small pores that are uniformly distributed in the amorphousphase and can be essentially spherical and can possess a smooth interiorsurface. The exterior surface of the material or particles can have asmooth and continuous exterior surface.

In more detail, the material of the present invention can include fromabout 0.01% by weight to about 99.9% by weight (based on the weight ofthe material) of the matrix or amorphous phase. The matrix or amorphousphase can include a silicon-containing oxide (e.g., silica), andoptionally at least one iron oxide; optionally at least one potassiumoxide; optionally at least one calcium oxide; optionally at least onesodium oxide; optionally at least one titanium oxide; and/or optionallyat least one magnesium oxide, or any combinations thereof. Preferably,the matrix or amorphous phase contains one or more or each of theseoptional oxides, such as in various amounts. Preferably, thesilicon-containing oxide is the major component by weight in the matrixor amorphous phase, such as where the silicon-containing oxide ispresent in an amount of at least 75% by weight, at least 85% by weight,at least 90% by weight, at least 95% by weight, at least 97% by weight,at least 98% by weight, at least 99% by weight (such as from 75% byweight to 99% by weight, from 90% by weight to 95% by weight, from 90%by weight to 97% by weight) based on the weight of the matrix oramorphous phase. Exemplary oxides that can be present in the amorphousphase include, but are not limited to, SiO₂, Al₂O₃, Fe₂O₃, Fe₃O₄, K₂O,CaO, Na₂O, TiO₂, and/or MgO. It is to be understood that, for purposesof the present invention, other metals and/or metal oxides can bepresent in the matrix or amorphous phase.

The material can include one or more minerals and/or ores, one or moreclays, and/or one or more silicates, and/or one more solid solutions.The minerals or ores can be aluminum-containing minerals or ores and/orsilicon-containing minerals or ores. These can be present asparticulates. These additional components can be present as at least onecrystalline particulate phase that can be non-continuous phase orcontinuous phase in the material. More specific examples include, butare not limited to, alumina, aluminum hydroxide, bauxite, gibbsite,boehmite or diaspore, ground cenosheres, perlite, stober silica, flyash, unreacted silica, silicate materials, quartz, feldspar, zeolites,bauxite and/or calcined clays. These components in a combined amount canbe present in the material in an amount, for instance, of from 0.001 wt% to 10 wt % or more, such as from 0.01 wt % to 10 wt %, 0.1 wt % to 8wt %, 0.5 wt % to 5 wt %, 0.75 wt % to 5 wt %, 0.5 wt % to 3 wt %, 0.5wt % to 2 wt % based on the weight of the material. These additionalcomponents can be uniformly dispersed throughout the matrix or amorphousphase (like filler is present in a matrix as discrete particulates). Asan option, the matrix or amorphous phase does not contain or containssignificantly low amounts of halides. For example, as an option, thematrix or amorphous phase contains 0.1 wt % or less of a halide (e.g.,F, Cl, Br, I) or a halide compound, such as 0.01 wt % or less, 0.001 wt% or less, or 0.0001 wt % or less, and preferably, 0 wt % of a halide,based on the weight of the amorphous phase. As an option, the matrix oramorphous phase contains low amounts or 0 wt % of fluorine, but with nolimitations on the amounts of the other halides. In this option, thefluorine amount can be the halide amount as mentioned above. It isunderstood that these weight percents are based on the elemental halide,such as elemental fluorine, and further based on the total weight of theamorphous phase or matrix.

The material can include at least one amorphous material that containsan entrapped vaporizable material such as water. As an option, theentrapped vaporizable material can be present in other parts of thematerial, such as in the crystalline phase, if present. The amorphousmaterial that contains an entrapped vaporizable material can have alower specific gravity than the other green body materials. Examples ofthe amorphous material include perlite, stober silica, pumice, andesite,scoria, volcanic glasses or any combination thereof. The amorphousmaterial that contains an entrapped vaporizable material, such as water,can be present by weight in the matrix or amorphous phase in an amountof at least 5% by weight, at least 10% by weight, at least 15% byweight, at least 20% by weight, at least 25% by weight, at least 30% byweight, at least 35% by weight, at least 40% by weight, such as from 5%by weight to 40% by weight, from 10% by weight to 35% by weight, from15% by weight to 25% by weight. The entrapped vaporizable material maycomprise vaporizable water (H₂O), carbon dioxide (CO₂), sulfur dioxide(SO₂), hydrogen sulfide (H₂S), nitrogen, argon, helium, neon, methane,carbon monoxide (CO), hydrogen, oxygen, hydrogen chloride (HCl),hydrogen fluoride (HF), hydrogen bromide (HBr), nitrogen oxide (NOx),sulfur hexafluoride (SF₆), carbonyl sulfide (COS), volcanic gases or anycombination thereof.

In the present invention, the matrix or amorphous phase can be acontinuous phase present in the material of the present invention.

One component that can be further present in the material or in thematrix or amorphous phase can be B₂O₃ and/or one or more transitionmetal oxides, such as Fe₂O₃, TiO₂, CoO, and/or NiO, or any combinationsthereof. The B₂O₃ and/or transition metal oxides can be present in thematrix or amorphous phase in various amounts, such as in low or traceamounts, for instance, 1 wt % or less, such as 0.5 wt % or less, such as0.25 wt % or less, such as 0.01 wt % to 0.001 wt %, based on the weightof amorphous phase.

As an option, one or more carbides, such as SiC and/or other forms canbe present in the material (e.g., sintered body). The carbide can bepresent as particles, particulates, and/or fibers and/or whiskers. As anoption, the carbide in part, or in its entirety, is not used as a poreformer, but is used as a particulate or fiber or whisker that remains aspart of the material (e.g. sintered body). This can be achieved, forinstance, by sintering in an inert atmosphere (and not an oxygencontaining atmosphere). This controlled sintering avoids the carbidereacting and forming a gas bubble. The carbide in this form can bepresent in any amount, such as from about 0.01% by weight to 25% byweight or more, based on the weight of the material (e.g. sinteredbody). The material (e.g., sintered body) can be a solid material (i.e.,no template or no hollow template in the interior) or the material canhave such a template. The material can be porous or non-porous. Thematerial can have microspheres (pre-formed or in situ formed) as anoption. The range of SiC particle size used in the green body materialcan have effects on both microsphere placement and/or size and strengthenhancement in the composite proppant product. The SiC or other carbidepowder used in the green body material should have a small size with alarge enough surface area to allow the oxidation to proceed as desired.SiC particles can have a particle size distribution with d_(fs) fromabout 0.5 to about 5.0 and from about 0.5 to about 1.5, wherein,d_(fs)={(d_(f90)−d_(f10))/d_(f50)} wherein d_(f10) is a particle sizewherein 10% (by volume) of the particles have a smaller particle size,d₁₅₀ is a median particle size wherein 50% (by volume) of the particleshave a smaller particle size, and d_(f90) is a particle size wherein 90%(by volume) of the particles have a smaller particle size. The medianparticle size, d_(f50), of the SiC is from about 0.01 μm to about 100 μmor from about 0.2 μm to about 5 μm, wherein d_(f50) is a median particlesize where 50% (by volume) of the particles of the distribution have asmaller particle size. The SiC can be present in an amount of from about0.01 to about 50 wt %, by weight of the green body or from about 0.01 toabout 10 wt %, by weight of the green body. The silicon carbide can havea surface area (BET) of from about 0.5 m²/g to about 100 m²/g or fromabout 8 m²/g to about 15 m²/g. These properties can remained in thesintered body as well or be within 10% or within 20% or within 40% ofthese parameters.

As an option, the matrix or amorphous phase can have no pores in thisphase. As an option, the matrix or amorphous phase can be porous.

With regard to the whiskers, the whiskers can be considered to be in theform of needles, for instance, as shown in some of the figures of thepresent invention. The whiskers can be mineral-based or metaloxide-based whiskers or can be considered whiskers formed of one or moreminerals and/or metal oxides. Preferably, the whiskers are mullitewhiskers (e.g., needle-shaped mullites). The whiskers can be silicatemineral whiskers or whiskers made of one or more silicate minerals.

The whiskers present in the material of the present invention can be orinclude in-situ whiskers, which, for purposes of the present invention,refer to the fact that the whiskers are formed during the formation ofthe material of the present invention (e.g., during formation of thecomposite of the present invention as a result of reactive sintering).The in-situ whiskers can have a different morphology from the pre-formedwhiskers. Preferably, the in-situ whiskers can have diameters of from0.05 micron to about 2 microns (e.g., from 0.05 micron to 2 microns,0.05 micron to 1.5 microns, 0.05 micron to 1 micron, 0.1 micron to 1micron, 0.5 micron to 1 micron, 0.75 micron to 1.5 microns). The in-situwhiskers can have an aspect ratio of from about 10 to about 100 (e.g.,from 10 to 75, from 15 to 100, from 20 to 100, from 10 to 45, from 15 to40, from 20 to 35). The in-situ whiskers have a length of from about 1micron to about 50 microns (e.g., from 1 micron to 40 microns, from 1micron to 30 microns, from 1 micron to 20 microns, from 1 micron to 10microns, from 1 micron to 5 microns, from 5 microns to 50 microns, from10 microns to 50 microns, from 15 microns to 50 microns, from 20 micronsto 50 microns, from 25 microns to 50 microns, and the like). It is to beunderstood that the in-situ whiskers can be a combination of variousdiameters, and/or various aspect ratios, and/or various lengths. It isto be understood that the in-situ whiskers can have relativelyconsistent diameters with varying aspect ratios and/or varying lengths.The in-situ whiskers can have relatively consistent aspect ratios andvarying diameters and/or varying lengths. The in-situ whiskers can haverelatively consistent lengths and varying diameters and/or varyingaspect ratios. With respect to consistent diameters and/or consistentaspect ratios and/or consistent lengths, it is to be understood, forpurposes of the present invention, that consistent refers to diameters,aspect ratios, and/or lengths that are within 25%, or within 10%, orwithin 5%, or within 1% of the other diameters and/or other aspectratios, and/or other lengths of the in-situ whiskers. The various rangesfor the diameters, aspect ratios, and/or lengths, for purposes of thepresent invention, can be considered average diameters, average aspectratios, and/or average lengths. As an option, these ranges can beconsidered maximum values for the diameters, and/or aspect ratios,and/or lengths.

The in-situ whiskers can be present in the material of the presentinvention in various amounts. For instance, the concentration of thein-situ whiskers can be present in an amount of from 0.1 wt % to 99.9 wt% based on the weight of the material Preferably, the concentration ofthe in-situ whiskers is present in an amount of from about 10 wt % toabout 50% (such as from 15% to 45 wt %, 20 wt % to 45 wt %, 30 wt % to45 wt %, 30 wt % to 40, and the like), based on the weight of thematerial.

The in-situ whiskers can be uniformly distributed throughout thematerial (e.g., uniform concentration) of the present invention. Thein-situ whiskers can be considered a continuous phase or can beconsidered a whisker phase in the material of the present invention. Thereference to continuous phase is a reference to in-situ whiskers thatcan, as an option, be present in such an amount that the in-situwhiskers contact or touch each other (in two or three dimensionsthroughout the material) and, therefore, form a continuous phasethroughout the material of the present invention. The concentration ofthe in-situ whiskers can be the same throughout the material or can bedifferent, such as in the form of gradients, wherein one region of thematerial can have a higher concentration of in-situ whiskers compared toanother region, such as a surface region versus a non-surface region.

Besides the in-situ whiskers or in lieu of the material of the presentinvention can include pre-formed whiskers, such as pre-formedmineral-based or metal oxide-based whiskers, such as mullite whiskers orneedle-shaped mullite that is pre-formed. The pre-formed whiskers canhave and preferably have a different morphology from the in-situwhiskers. For instance, the pre-formed whiskers can be micro-,sub-micro, or nano-whiskers. The pre-formed whiskers can be whiskerseeds. The pre-formed whiskers can have an aspect ratio of from 1 to 20,such as 1 to 5. The pre-formed whiskers can have a length of from 0.01to 1 micron, such as from 0.01 to 0.75 micron, from 0.1 to 0.875 micron,from 0.1 to 0.5 micron. The pre-formed whiskers can have a diameter offrom about 0.01 to 0.5 micron (e.g., from 0.01 to 0.3 micron). Thepre-formed whiskers can be present in the material of the presentinvention in an amount of from about 0.001 wt % to 40 wt %, such as from0.001 wt % to 30 wt %, from 0.001 wt % to 20 wt %, from 0.001 wt % to 10wt %, from 0.01 wt % to 5 wt % or less based on the weight of thematerial of the present invention. The pre-formed whiskers can beuniformly present throughout the material. The pre-formed whiskers canbe present as a non-continuous phase. The pre-formed whiskers can bescattered in such a manner that the pre-formed whiskers do not toucheach other or rarely do.

The in-situ whiskers and/or pre-formed whiskers can be present in arandom manner throughout the matrix or amorphous phase of the materialof the present invention. The whiskers can be considered to be in arandom alignment in the material of the present invention.

The material of the present invention can be in the form of a sphere,where this sphere is solid or hollow, or has one or more voids presentwithin the sphere. The material can be a sphere or similar shape, whichis hollow in the interior of the sphere. The sphere can have a smooth,glassy surface that can contribute to an effective seal againstpenetration from liquids or gases in the environment.

As an option, the material of the present invention can form a shellaround one or more other materials, such as a template or templatematerial, which can be in the form of a sphere or other shape and whichcan be a solid material or a hollow material. For instance, the materialof the present invention can form a shell around a hollow sphere, suchas a cenosphere or other similar material. When the material of thepresent invention is present as a shell and encapsulates one or moreother materials, such as a sphere (like a hollow sphere), thecoefficient of thermal expansion between the shell and the templatematerial can be the same or within 20% of each other, such as within10%, within 5%, within 1%, or within 0.5% of each other.

As an option, the present invention relates to a particle or proppanthaving a template material and shell on the template material, whereinthe shell at least includes or is the material of the present inventionas described herein. The template material (for instance, a hollowsphere, such as a cenosphere) can have the same components as the shellfrom the standpoint of having a matrix or amorphous phase that haswhiskers or a whisker phase present. For purposes of the presentinvention, it is to be understood that the template material can havethe same or different composition and/or characteristics as the shellwith respect to the components present and/or amount of each component.The concentration of the in-situ and/or pre-formed whiskers in thetemplate can be different from the concentration of the in-situ and/orpre-formed whiskers in the shell. For instance, the weight ratio of theconcentration of whiskers present in the shell to the concentration ofwhiskers in the template can be a weight ratio of 50:1, 40:1, 30:1,25:1, 20:1, 15:1, 10:1, 7:1, 5:1, 4:1, 3:1, 2:1, 1.75:1, 1.50:1, 1.25:1(shell:template), and the like. For instance, the concentration of thewhiskers present in the shell can be the amounts referenced above forthe material of the present invention and the amount of the whiskerspresent in the template can be, for instance, from about 0.1 wt % to 40wt %, such as from 0.5 wt % to 30 wt %, 0.75 wt % to 20 wt %, 1 wt % to10 wt %, 1 wt % to 5 wt %, 1 wt % to 3 wt %, wherein this weight isbased on the weight of the template material. The exact composition ofthe shell compared to the template material can be the same or differentwith respect to the individual components that make up the shell andtemplate.

For purposes of the present invention, the in-situ whisker concentrationto pre-formed whisker concentration can be a weight ratio such as from1000:1, 100:1, 75:1, 50:1, 40:1, 25:1, 10:1, 200:1, 150:1, 100:1, 50:1,25:1, 10:1, 5:1, 4:1, 3:1, 2:1, 1.75:1, 1.5:1, 1.25:1, 1.1:1, 1:1,0.9:1, 0.8:1 (in-situ:pre-formed), and the like, wherein this weightratio is based on the weight percentage of in-situ whiskers compared tothe weight percent of pre-formed whiskers present in the material of thepresent invention (or in the template).

The proppant (or sintered body) can have a hollow core or a solid core,and can have a low specific gravity, for example, a specific gravity ina range of from about 1.0 g/cc to about 2.5 g/cc, while maintaining acrush strength in a range of from about 500 psi to about 20,000 psi,and/or a flexural strength in a range of from about 1 MPa to about 200MPa, or more.

The proppants of the present invention can provide oil and gas producerswith one or more of the following benefits: improved flow rates,enhanced hydrocarbon recovery, improved productive life of wells,improved ability to design hydraulic fractures, and/or reducedenvironmental impact. The proppants of the present invention aredesigned to improve flow rates, eliminating or materially reducing theuse of permeability destroying polymer gels, reducing pressure dropthrough the proppant pack, and/or reducing the amount of water trappedbetween proppants thereby increasing hydrocarbon “flow area.” Lowerdensity enhances proppant transport deep into the formation, increasingthe amount of fracture-area propped, and thereby increasing themechanical strength of the reservoir. The low density of the presentinvention's proppants can reduce transportation costs. Because theproppant is lighter, less pumping force is needed, potentially loweringproduction costs and reducing damage to the formation.

Proppants of the present invention preferably enable the use of simplercompletion fluids and less (or slower) destructive pumping. Formationspacked with lower density proppants of the present invention can exhibitimproved mechanical strength/permeability and thus increased economiclife. Enhanced proppant transport enabled by lower density proppantsenable the emplacement of the proppant of the present invention in areasthat were previously impossible, or at least very difficult to prop. Asa result, the mechanical strength of the subterranean formations can beimproved with reduced decline rates over time.

If lower density proppants are employed, water and/or brine solutionscan be used in place of more exotic completion fluids. The use ofsimpler completion fluids can reduce or eliminate the need to employde-crosslinking agents. Further, increased use of environmentallyfriendly proppants can reduce the need to employ other environmentallydamaging completion techniques such as flashing formations withhydrochloric acid. The low density properties that can be exhibited bythe proppants of the present invention eliminates or greatly reduces theneed to employ permeability destroying polymer gels as the proppants aremore capable of staying in suspension.

The present invention also relates to low density proppants that can beutilized, for example, with water and/or brine carrier solutions.

The proppant can be either solid throughout or hollow within theproppant. In the present invention, a solid proppant is defined as anobject that does not contain a void space in the center, although aporous material would be suitable and is optional; a fully densematerial is not a requirement of solid proppant. A hollow material isdefined as an object that has at least one void space inside (e.g.,generally centrally located within the proppant) with a defined size andshape.

The material of the present invention can have isotropic propertiesand/or anisotropic properties. In other words, the ceramic material canhave measurable properties that are identical in all directions(isotropic), but can also have properties that differ according to thedirection of measurement (anisotropic).

The template preferably can have a diameter in the size range of, forexample, from about 1 nm to about 3000 μm, or from about 25 μm to about2000 μm, or from about 80 μm to about 1500 μm, or from about 120 μm toabout 300 μm.

The proppants of the present application can have a specific gravity of,for example, from about 0.6 g/cc to about 2.5 g/cc. The specific gravitycan be, for example, from about 2.0 g/cc to about 2.5 g/cc, from about1.0 g/cc to about 2.5 g/cc, from about 1.0 g/cc to about 2.2 g/cc, fromabout 1.0 g/cc to about 2.0 g/cc, from about 1.0 g/cc to about 1.8 g/cc,from about 1.0 to about 1.6 g/cc, or from about 0.8 g/cc to about 1.6g/cc. Other specific gravities above and below these ranges can beobtained. The term “specific gravity” as used herein is the weight ingrams per cubic centimeter (glee) of volume, excluding open porosity indetermining the volume. The specific gravity value can be determined byany suitable method known in the art, such as by liquid (e.g., water oralcohol) displacement or with an air pycnometer.

The strength properties of the proppant can be dependent on theapplication. The proppant can have a crush strength of at least 1,000psi. The crush strength can be from about 2,000 psi to about 25,000 psior higher. The crush strengths can be greater than 9,000 psi, greaterthan 15,000 psi, or greater than 25,000 psi. Other crush strengths belowor above these ranges are possible. A crush strength below 3000 psi isan option, such as 500 psi to 3000 psi, or 1000 psi to 2,000 psi. Crushstrength can be measured, for example, according to American PetroleumInstitute Recommended Practice 60 (RP 60).

The proppant can have any particle size. For instance, the proppant canhave a particle diameter of from about 1 nm to 1 cm, from about 1 μm toabout 1 mm, from about 10 μm to about 10 mm, from about 100 μm to about5 mm, from about 50 μm to about 2 mm, or from about 80 μm to about 1,500μm. The optimum size of the proppant can depend on the particularapplication.

The clay or clays used can be in uncalcined, partially calcined, and/orcalcined forms, or any mixtures of such forms. The term “uncalcinedclay” is understood by those of ordinary skill in the art to mean clayin its natural “as-mined” condition. Uncalcined clay has not beensubjected to any type to treatment that would result in a chemical ormineralogical change, and can also be referred to as “raw” clay. Theterms “partially calcined clay” and “calcined clay” are understood bythose of ordinary skill in the art to mean clay that has been subjectedto a heat treatment at times and temperatures, typically about 500° C.to 800° C., to remove some (partially calcined) or substantially all(calcined) organic material and water of hydration from the clay.

In the present invention, the present invention also relates to aproppant used to prop open subterranean formation fractions comprising aparticle or particles with controlled buoyancy and/or crush strength.The controlled buoyancy can be a negative buoyancy, a neutral buoyancy,or a positive buoyancy in the medium chosen for pumping the proppant toits desired location in the subterranean formation. The medium chosenfor pumping the proppant can be any desired medium capable oftransporting the proppant to its desired location including, but notlimited to a gas and/or liquid, energized fluid, foam, and aqueoussolutions, such as water, brine solutions, and/or synthetic solutions.Any of the proppants of the present invention can have a crush strengthsufficient for serving as a proppant to prop open subterranean formationfractures.

The proppants of the present invention can comprise a single particle ormultiple particles and can be a solid, partially hollow, or completelyhollow in the interior of the particle. The particle can be spherical,nearly spherical, oblong (or any combination thereof), or have othershapes suitable for purposes of being a proppant. The proppant maycontain filler in addition to the whiskers. The filler is a compoundthat does not reactively sinter with the ceramic material. Examples offillers include graphite, metals (e.g., noble metals), metal oxides(e.g., cerium oxide) and metal sulfides (e.g., molybdenum disulfide).

The proppant of the present invention can be a sintered body, such as asphere having a Krumbein sphericity of at least about 0.5 and aroundness of at least about 0.4. The proppant can include a) a pluralityof ceramic whiskers or oxides thereof and b) a glassy phase and c)optionally at least one crystalline phase, such as a non-whiskercrystalline phase and d) optionally a plurality of microspheres, whereinthe sintered sphere has a diameter of from about 90 microns to 2,500microns, and the sintered sphere has a specific gravity of from 0.8 g/ccto about 3.8 g/cc, and the proppant has a crush strength of from about1,000 psi or greater.

The proppants described herein of the present invention can include oneor more of the following characteristics:

-   -   1) said glassy phase is present in an amount of at least 10% by        weight, based on the weight of the proppant (e.g., at least 15%,        at least 20%, at least 25%, at least 30%, at least 40%, at least        50%, such as from 15% to 70%, all based on wt %, based on the        weight of the proppant);    -   2) said whiskers, such as ceramic whiskers, have an average        length of less than 5 microns (e.g., less than 4 microns, less        than 3.5 microns, less than 3.2 microns, less than 3 microns,        less than 2.7 microns, less than 2.5 microns, less than 2.2        microns, such as from 0.5 micron to 5 microns, or from 1 micron        to 3.5 microns, or from 0.8 micron to 3.2 microns, or from 1        micron to 3 microns or from 1.2 to 1.8 microns);    -   3) said whiskers, such as ceramic whiskers, have an average        width of less than 0.35 micron (e.g., less than 0.3, less than        0.28, less than 0.25, less than 0.2, less than 0.15, such as        from 0.05 to 0.34 micron, from 0.2 to 0.33 micron, from 0.1 to        0.3 micron, from 0.12 to 0.2 micron, all units in microns);    -   4) said whiskers, such as ceramic whiskers, have a whisker        length distribution, d_(as), of about 8 or less (e.g., 7 or        less, 6 or less, 5 or less, 4 or less, 3 or less, 2 or less, 1        or less, 0.5 or less, 0.4 or less, 0.3 or less, 0.2 or less,        such as 0.1 to 8, 0.1 to 7, 0.1 to 6, 0.1 to 5, 0.1 to 4, 0.1 to        3, 0.1 to 2, 0.1 to 1, 0.1 to 0.75, 0.1 to 0.5, 0.1 to 0.3, 0.1        to 0.2, 0.1 to 1.8), wherein, d_(as)={(d_(a90)−d_(a10))/d_(a50)}        wherein d_(a10) is a whisker length wherein 10% of the whiskers        have a smaller length, d_(a50) is a median whisker length        wherein 50% of the whiskers have a smaller whisker length, and        d_(a90) is a whisker length wherein 90% of the whiskers have a        smaller whisker length;    -   5) said proppant having a porosity from about 1% to 70% by        weight where

${{porosity}\mspace{14mu}(\%)} = {100 - {\left( \frac{{SG}_{m}}{{SG}_{t}} \right) \times 100\mspace{14mu}{and}}}$SG_(m) = measured  specific  gravity  andSG_(t) = theoretical  specific  gravity;

-   -   6) said proppant having a porosity from about 5 wt % to 50 wt %        by weight of proppant;    -   7) said proppant having a porosity from about 3 wt % to 20 wt %        by weight of proppant;    -   8) said proppant having a porosity from about 4 wt % to 16 wt %        by weight of proppant;    -   9) said proppant having a specific gravity of from 1.6 to 1.8        with a crush strength of at least 2000 psi;    -   10) said proppant having a specific gravity of from 1.8 to 2        with a crush strength of at least 3000 psi;    -   11) said proppant having a specific gravity of from 2 to 2.1        with a crush strength of at least 5,000 psi;    -   12) said proppant having a specific gravity of from 2.25 to 2.35        with a crush strength of at least 8,000 psi;    -   13) said proppant having a specific gravity of from 2.5 to 3.2        with a crush strength of at least 18,000 psi;    -   14) said proppant having a specific gravity of from 2.5 to 3.2        with a crush strength of at least 25,000 psi;    -   15) said proppant having a combined clay amount and cristobalite        amount of less than 20% by weight of proppant;    -   16) said proppant having a free alpha-alumina content of at        least 5% by weight of said proppant (e.g., 5 wt % to 70 wt % or        more, at least 10 wt %, at least 20 wt %, at least 30 wt %, at        least 40 wt %, based on the weight of the proppant);    -   17) said proppant having an HF etching weight loss of less than        35% by weight of said proppant (e.g., less than 30% by weight,        less than 25% by weight, less than 20% by weight, less than 15%        by weight, less than 10% by weight, such as from 10 wt % to 34        wt %, from 15 wt % to 30 wt %, from 18 wt % to 28 wt % by weight        of said proppant);    -   18) said proppant having said microspheres present as hollow        glass microspheres having a particle size distribution, d_(as),        of from about 0.5 to about 2.7 (e.g., 0.5 to 2.6, 0.8 to 2.2, 1        to 2, 0.5 to 2, 0.5 to, 1.5, 0.5 to 1), wherein,        d_(as)={(d_(a90)−d_(a10))/d_(a50)} wherein d_(a10) is a particle        size wherein 10% (by volume) of the particles have a smaller        particle size, d_(a50) is a median particle size wherein 50% (by        volume) of the particles have a smaller particle size, and        d_(a90) is a particle size wherein 90% (by volume) of the        particle volume has a smaller particle size;    -   19) said proppant having microspheres present wherein said        microspheres are uniformly present in said proppant or in a        layered region of said proppant;    -   20) said whiskers, such as ceramic whiskers, are present in an        amount of from 5% to 60% by weight of said proppant (e.g., from        5% to 50%, from 5% to 45%, from 5% to 40%, from 5% to 35%, from        5% to 30%, from 5% to 25%, from 5% to 20%, from 5% to 15%, from        10% to 25%, from 15% to 25%, all by wt % based on the weight of        said proppant);    -   21) said proppant has a combined clay amount and cristobalite        amount of less than 20% (e.g., less 15%, less than 10%, less        than 5%, less than 1%, such as from 0.1% to 3%, all wt %) by        weight of proppant and/or said whiskers, such as mullite        whiskers, are present in an amount of 60% or more by weight of        said proppant (e.g., from 5% to 50%, from 5% to 45%, from 5% to        40%, from 5% to 35%, from 5% to 30%, from 5% to 25%, from 5% to        20%, from 5% to 15%, from 10% to 25%, from 15% to 25%, all by wt        % based on the weight of said proppant);    -   22) said proppant has a high whisker distribution density based        on individual whiskers present in the proppant (# of whiskers        per mg of proppant);    -   23) said proppant has a unimodal whisker distribution;    -   24) said proppant has at least two layers that form a laminate        structure (such as three layers or four layers or five layers);    -   25) said proppant has at least a first layer and a second layer        that form a laminate structure wherein the average length of        said whiskers in said first layer compared to said second layer        is different;    -   26) said proppant has at least a first layer and a second layer        that form a laminate structure wherein the average width of said        whiskers in said first layer compared to said second layer is        different;    -   27) said proppant has a smooth surface comprised of both        crystalline and amorphous materials.    -   28) said whiskers in said proppant are less euhedral and more        anhedral;    -   29) said proppant has at least one region (e.g., one        circumference or radial region closer to the outer surface) of        first whiskers and at least one region (e.g., one circumference        or radial region further away from the outer surface of the        proppant) of second whiskers, wherein the average whisker length        for the first and second whiskers is different by at least 10%        (e.g., at least 15%, at least 20%, at least 25%, at least 30%,        at least 35%, at least 40%, at least 45%, at least 50%, at least        55%, at least 60%, at least 70%, at least 80%, at least 90%, at        least 100%, at least 120%, at least 140, at least 200%        different). Where R is the radius of the proppant and R=0 is the        center of the proppant, and R=100 is the outer surface of the        proppant, an R from 1 to 50 (e.g., from 5 to 50, from 10 to 40,        from 10 to 25, from 10 to 20) can have an average whisker length        that is larger than a region where R is greater (an outer        region). These regions can be all interior to a shell region if        present, or one of the regions can be part of a shell region and        the other region can be part of a template or core region that        is encapsulated by a shell(s).    -   30) said proppant has at least one radial region of first        whiskers and at least one region of second whiskers, wherein the        average whisker width is different by at least 10% (e.g., at        least 15%, at least 20%, at least 25%, at least 30%, at least        35%, at least 40%, at least 45%, at least 50%, at least 55%, at        least 60%, at least 70%, at least 80%, at least 90%, at least        100%, at least 120%, at least 140, at least 200% different).        Where R is the radius of the proppant and R=0 is the center of        the proppant, and R=100 is the outer surface of the proppant, an        R from 1 to 50 (e.g., from 5 to 50, from 10 to 40, from 10 to        25, from 10 to 20) can have an average whisker width that is        larger than a region where R is greater (an outer region).    -   31) said proppant has a major phase of whiskers of less than one        micron and a secondary minor phase of whiskers of one micron or        higher; and/or    -   32) said whiskers, such as ceramic whiskers, have a whisker        length distribution having d_(a90), which is a whisker length        wherein 90% of the whiskers have a smaller whisker length, of        less than 12 microns (e.g., less than 10 microns, less than 8        microns, less than 7 microns, less than 6 microns, less than 5        microns, less than 4 microns, less than 3 microns, less than 2        microns, such as from 1 to 10, 1.5 to 5, 1.7 to 5, 1.8 to 4, 1.9        to 3.5, 1.5 to 3.5).

It is to be understood that all averages and distributions mentionedabove or herein are based on measuring at least 50 whiskers picked on arandom basis in a proppant. Preferably, at least 10 proppants aremeasured in this manner and an average obtained.

In the present invention, the one or more of said characteristicsmentioned above can provide stress reducing properties on said proppantcompared to the same proppant but without said characteristics. Theproppant can have an alumina content of at least 35% by weight of saidproppant, such as at least 40%, at least 45%, at least 50%, at least55%, at least 60%, such as from 35% to 55%, all wt % based on the weightof the proppant. The proppant can have mullite whiskers, such as presentin an amount of from 10 wt % to 40 wt % by weight of said proppant(e.g., 15 wt % to 30 wt %, or 20 wt % to 25 wt % and the like). Theproppant can include quartz. The proppant can have quartz in an amountof from 0.1 wt % to 1 wt % based on the weight of the proppant. Theproppant can have at least one layered shell encapsulating a hollowspherical template. The proppant can have at least one layered shellencapsulating a hollow spherical template, and said microspheres (e.g.,in-situ formed and/or pre-formed) are present in said at least layeredshell.

The present invention also relates to a method of preparing a proppantthat employs reactive or reaction sintering to form a uniquemicrostructure, in which anisotropic crystals (such as whiskers,needles, leaves, or fibers) are formed in-situ through the chemicalreactions of the raw or starting materials. The raw materials cancomprise ceramic precursors, for example, talc, clay, alumina, silica,kyanite, or any combination thereof. The reactive sintering process canproduce a ceramic proppant having randomly aligned whiskers that can beformed in-situ and/or introduced through one or more of the startingmaterials. The method can produce a ceramic proppant comprisinganisotropic crystals. The flexural strength of such a proppant can have,for example, at least 50% more strength than a proppant with isotropicstructure that can be formed by sintering pre-formed materials, at thesame or about the same specific gravity. The reactive or reactionsintering can form substantially spherical microspheres or pores with asmooth glassy interior wall inside the amorphous phase by controlledrelease of gaseous materials, such as water, from the at least oneamorphous material that contains an entrapped vaporizable material suchas water. The reactive or reaction sintering process in the presence ofthe amorphous material that contains an entrapped vaporizable materialsuch as water, promotes a smooth outer surface on the proppant. Theamorphous material that contains an entrapped vaporizable material, suchas water, can comprise perlite, stober silica, pumice, andesite, scoria,volcanic glasses or any combinations thereof.

In the present invention, the ceramic particles or any type of proppantparticle can benefit from using membrane separation processes for one ormore of the starting materials that are used to form the ceramicparticles or any type of proppant. The membrane separation processes canbe also useful in the final product as well.

The starting material(s) particle size and its distribution can bestrictly controlled by membrane separation processes. The selectedincoming raw materials can be dispersed into a slurry, such as anaqueous slurry like water. At least one dispersant can be used as wellfor improving the dispersion of the slurry. The slurry can be milled,such as through an attrition mill, ball mill, jet mill, hammer mill orany combination thereof. After milling or otherwise obtaining thedesired general particle size, the slurry can be diluted to a desirableconcentration, then feed into at least one membrane filtration device.By such a process, the larger particles are left in the filtration cakeor in the retained slurry while the smaller particles remain in theeffluent slurry. With such a process, the larger particles are filteredout. The effluent slurry can be then feed in to a second membrane filterwith a smaller pore size. Going through the same process as describedabove, the filtration cake or the retained slurry having a narrowparticle size distribution of raw materials is obtained. Essentiallythis membrane process permits a very accurate and controlled way toobtain a “cut” of desirable particle sizes, whereby the unwanted smallerparticles and the unwanted larger particles are removed.

In the present invention, one can use the above membrane filtrationprocess to separate particles size into various groups, such as with anaverage particle size of 0.2 micron, 0.5 micron, 1 micron, 1.5 micron,and 2.0 microns, and so on, depending on the membrane pore size. Thewidth of the size distribution can be determined by the two “cuts” ofmembrane sizes. In general, a much narrower size distribution isdesirable for product performance and this process permits such adistribution. In addition to membrane filtration, air classification canbe employed alone or in combination with membrane separation to producethe desired mean particle size and/or size distribution.

As an example, raw material particles with the same particle sizedistributions can be mixed, and then spray coated to form ceramic greenspheres, or granulated in a granulator. Due to the same particle sizes,particle packing is well controlled. Pores between particles can be wellpreserved. During the firing process, particles sinter together, and theporosity can be well preserved after the firing process, with a narrowpore size distribution. By controlling the particle size with the narrowdistribution, a pore size can be well controlled after the sinteringprocess. Narrow pore size distribution can be achieved, so that anadequate amount of porosity can be added in to the ceramics, while mostof mechanical strength can be preserved.

As a further example, two different size cuts of raw materials can bemixed together (e.g., 2 micron particles mixed with 0.5 micron particlesand 0.2 micron particles), going through the forming processes describedabove. After forming, the green body can be subjected to firing at ahigh temperature, and a near zero porosity containing proppant can beproduced.

In the present invention, two types of a membrane separation device canbe used (e.g., a “dead end filtration” and another type is cross flowmembrane separation.) The former one can handle a relatively highconcentration of slurry, which yield a broader particle sizedistribution. The later gives very narrow and clean cut particles sizedistribution.

In the present invention, size control of the raw or starting material,provide the possibility of precise sintering under well controlledfiring cycles. So the grain size growth can be controlled, and highstrength materials with uniform small grain size materials can beproduced under the same specific gravity.

In the present invention, the number of pores and/or pore size can bewell controlled, so an adequate amount of porosity can be added into aceramic proppant, while loss of mechanical strength can be minimized.Therefore, high strength/low specific gravity proppant can be produced.Formation of pores can be enhanced by the addition of an amorphousmaterial that contains an entrapped vaporizable material such as waterand releasing the vaporizable material in a controlled rate.

As an option, in the present invention, the various average particlesizes and/or particle size distributions are the same or about the samewith respect to each of the starting materials that form the green body.When the particle sizes of one or more, and, preferably all of thestarting materials that can have particle sizes, are about the same orthe same, the formation of the green body by mixing the various startingmaterials together can be more uniform and the distribution of thedifferent starting materials gets distributed throughout the green bodyin a more uniform way, such that the overall green body and theresulting sintered body, such as the proppant, has a uniformdistribution of each of the starting materials, thereby forming a veryconsistent sintered body having consistent properties throughout thesintered body or selected parts or regions thereof, and thereby reducingthe chances of a flaw or defect existing in the sintered body. Theaverage particle size and/or distribution of two or more of the startingmaterials can be within +/−20% of each other, +/−15% of each other,+/−10% of each other, +/−7% of each other, +/−5% of each other, +/−4% ofeach other, +/−3% of each other, +/−2% of each other, +/−1% of eachother, +/−0.75% of each other, +/−0.5% of each other, +/−0.25% of eachother, +/−0.1% of each other, +/−0.05% of each other, or +/−0.01% ofeach other.

As a result of such techniques, such as the membrane filtration device,the particle size distribution for any of the starting materials, suchas the ceramic or ceramic precursor, the microsphere former, metaloxide, metals, (or, for that matter, any particulate starting material)and the like can have a particle distribution that is very tight, suchthat the particle size distribution as defined herein(d=[(D₉₀−D₁₀)/D₅₀], wherein d is 0.4 to 1, such as 0.05 to 0.9, 0.07 to0.5, 0.09 to 0.4, and the like.

The expression “reactive sintering” as used herein, can include aprocess wherein heat is applied to a composition, causing thatcomposition to undergo, at least in part, a chemical reaction forming anew composition. The composition is heated to below or about its meltingpoint.

The term “green body” or “green pellet” refers to pre-sintered materialof this invention that has been shaped from the disclosed compositionsbut are not sintered. The mixing step typically provides an aqueousdispersion or paste, which is later dried.

Drying can be performed at a temperature in the range of from about 30°C. to 600° C., such as from about 120° C. to 150° C., and can occur overa period of up to about 48 hours, depending on the drying techniqueemployed. Any type of dryer customarily used in the industry to dryslurries and pastes can be used. Drying can be performed in a batchprocess using, for example, a stationary dish or container.Alternatively, drying can be performed in a spray dryer, fluid beddryer, rotary dryer, rotating tray dryer or flash dryer. The pellets canbe screened to provide a suitable median particle size, preferably afterdrying. For example, a top screen having a mesh size of about 10 or 11mesh can be used to screen out the largest particles and a bottom screenhaving a mesh size of about 18 or 20 can be used to remove the finerparticles. The choice of top and bottom screens depends, in part, on themixture produced and can be adjusted to tailor the median particle sizeof the mixture. A further screening may take place after sintering. Theslurry containing the green body material to form the green body can besprayed or otherwise applied to a hot plate(s) (horizontal or inclinedsurface). The hot plate can have a metal or ceramic surface. A burner ora series of burners are located under the plate to provide heat to thehot plate surface. The surface is maintained above the evaporationtemperature of the solvent (e.g., water) and preferably a lot higher(e.g., at least 10% higher or at least 30% or at least 50% higher intemperature). The droplet sizes are bigger in size than the desireddried size. For instance, the droplet size can be at least 10% larger,at least 50%, at least 100% larger than the final granule size thatforms after evaporation occurs. The process/device described in U.S.Pat. No. 5,897,838 (incorporated in its entirety by reference herein)can be adopted as well for this purpose.

The template material can be porous, non-porous, or substantiallynon-porous. For purposes of the present invention, a substantiallynon-porous material is a material that is preferably at least 80 vol %non-porous in its entirety, more preferably, at least 90 vol %non-porous. The template material can be a hollow sphere or it can be aclosed foam network, and/or can be a non-composite material. Anon-composite material, for purposes of the present invention, is amaterial that is not a collection of particles which are bound togetherby some binder or other adhesive mechanism. The template material of thepresent invention can be a single particle. The template material can bea cenosphere or a synthetic microsphere such as one produced from ablowing process or a drop tower process.

The template material can have a crush strength of 5000 psi or less,3000 psi or less, or 1000 psi or less. In the alternative, the templatematerial can have a high crush strength such as 1000 psi or more, orfrom about 3000 psi to 10,000 psi. For purposes of the presentinvention, crush strength can be determined according to API Practice 60(2^(nd) Ed. December 1995). A template material having a low crushstrength can be used to provide a means for a coating to be applied inorder to form a shell wherein the shell can contribute a majority, ifnot a high majority, of the crush strength of the overall proppant.

The proppant can be spherical, oblong, nearly spherical, or any othershapes. For instance, the proppant can be spherical and have a Krumbeinsphericity of at least about 0.5, at least 0.6, at least 0.7, at least0.8, or at least 0.9, and/or a roundness of at least about 0.4, at least0.5, at least 0.6, at least 0.7, or at least 0.9. The term “spherical”refers to sphericity and roundness on the Krumbein and Sloss Chart byvisually grading 10 to 20 randomly selected particles.

In accordance with the method of the present invention, the ceramicproppant produced as described above may be used as proppants, gravel orfluid loss agents in hydraulic fracturing and/or frac packing. As statedabove, the present invention also relates to a proppant formulationcomprising one or more proppants of the present invention with acarrier. The carrier can be a liquid or gas or both. The carrier can be,for example, water, brine, hydrocarbons, oil, crude oil, gel, foam, orany combination thereof. The weight ratio of carrier to proppant can befrom 10,000:1 to 1:10,000, or any ratio in between, and preferably about0.1 g proppant/liter fluid to 1 kg proppant/liter fluid.

The present invention, as one example, relates to a method for producingthe material of the present invention as stated herein. The startingcomponents used in the methods described herein can be the samecomponents or precursors of the same components mentioned earlier.

The present invention also relates to a method to make strong, tough,and/or lightweight glass-ceramic matrix composites through aself-toughening structure generated by viscous reaction sintering of acomplex mixture of oxides. For purposes of this invention, glass-ceramiccomposite can be a material in which glass can comprise from about 0.01%by weight to about 99.9% by weight, based on the weight of thecomposite. The typical composition of the starting mixture can includethe following oxides and/or their precursors in one form or another:Al₂O₃, SiO₂, Fe₂O₃, Fe₃O₄, K₂O, CaO, Na₂O, TiO₂, and MgO.

The method can include forming a green body. The green body can beformed from a green body material that includes:

-   -   i. at least one metal oxide(s) (and preferably at least two        different metal oxides, a first metal oxide and a second metal        oxide that is different from the first metal oxide) that is        preferably capable of forming whiskers in-situ. The metal oxide        can be an aluminum oxide and/or an aluminum bearing mineral (or        ore) and/or a silicon oxide or a silicon bearing mineral (or        ore) or precursors thereof, and    -   ii. at least one amorphous material containing at least one        entrapped vaporizable material such as water, and    -   iii. pre-formed whiskers (e.g., ceramic or metal oxide or        mineral based whiskers), and    -   iv. at least one whisker promoter, preferably in the absence of        halide or halide compounds, or preferably in the absence of        fluorine or fluorine compounds.

The green body is then subjected to sintering under sintering conditionsto form in-situ the material of the present invention (e.g., a compositehaving at least one whisker phase and at least one amorphous phase andoptionally, at least one crystalline particulate phase).

The at least one metal oxide or precursor thereof can have any particlesize distribution. For example, the particle size distribution, d_(as),can be from about 0.5 to about 15, wherein,d_(as)={(d_(a90)−d_(a10))/d_(a50)} wherein d_(a10) is a particle sizewherein 10% (by volume) of the particles have a smaller particle size,d_(a50) is a median particle size wherein 50% (by volume) of theparticles have a smaller particle size, and d_(a90) is a particle sizewherein 90% (by volume) of the particle volume has a smaller particlesize. The d_(as) can be from 0.5 to 15, 0.75 to 15, 1 to 15, 1 to 5, 1to 6, 1 to 8, 5 to 15, 0.5 to 10, 0.5 to 5, and the like. The metaloxide(s) or metal oxide precursor can have a median particle size,d_(a50), of from about 0.01 μm to about 100 μm, wherein d_(a50) is amedian particle size where 50% (by volume) of the particles of thedistribution have a smaller particle size. The median particle size,d_(a50), can be from about 1 μm to about 5 μm, from 1 to 5 μm, 1 to 90μm, 1 to 80 μm, 1 to 70 μm, 1 to 60 μm, 1 to 50 μm, 1 to 40 μm, 1 to 30μm, 1 to 20 μm, 1 to 10 μm, 10 to 90 μm, 20 to 80 μm, 30 to 70 μm, andthe like, wherein d_(a50) is a median particle size where 50% (byvolume) of the particles of the distribution have a smaller particlesize.

When preferably two different metal oxides (or precursor thereof) areused, the second metal oxide (or precursor thereof) can have anyparticle size, such as a particle size distribution, The d_(as) can befrom 0.5 to 15, 0.75 to 15, 1 to 15, 1 to 5, 1 to 6, 1 to 8, 5 to 15,0.5 to 10, 0.5 to 5d_(ss), of from about 0.5 to about 15, wherein,d_(ss)={(d_(s90)−d_(s10))/d_(s50)} wherein d_(s10) is a particle sizewherein 10% (by volume) of the particles have a smaller particle size,d_(s50) is a median particle size wherein 50% (by volume) of theparticles have a smaller particle size, and d_(s90) is a particle sizewherein 90% (by volume) of the particle volume has a smaller particlesize. The d_(as) can be from 0.5 to 15, 0.75 to 15, 1 to 15, 1 to 5, 1to 6, 1 to 8, 5 to 15, 0.5 to 10, 0.5 to 5 and the like. The secondmetal oxide (or precursor thereof) can have a median particle size,d_(a50), of from about 0.01 μm to about 100 μm, wherein d_(a50) is amedian particle size where 50% (by volume) of the particles of thedistribution have a smaller particle size. The median particle size,d_(a50), can be from about 1 μm to about 5 μm, from 1 to 5 μm, 1 to 90μm, 1 to 80 μm, 1 to 70 μm, 1 to 60 μm, 1 to 50 μm, 1 to 40 μm, 1 to 30μm, 1 to 20 μm, 1 to 10 μm, 10 to 90 μm, 20 to 80 μm, 30 to 70 μm, andthe like, wherein d_(a50) is a median particle size where 50% (byvolume) of the particles of the distribution have a smaller particlesize.

As an option, the particle size distribution and/or the median particlesize of the first metal oxide or precursor thereof and the second metaloxide or precursor thereof can be the same or different, or can bewithin 1%, 5%, 10%, 15%, 20%, 25% of each other.

The at least one amorphous material containing at least one entrappedvaporizable material such as water can have any particle sizedistribution. For example, the particle size distribution, d_(as), canbe from about 0.5 to about 15, wherein,d_(as)={(d_(a90)−d_(a10))/d_(a50)} wherein d_(a10) is a particle sizewherein 10% (by volume) of the particles have a smaller particle size,d_(a50) is a median particle size wherein 50% (by volume) of theparticles have a smaller particle size, and d_(a90) is a particle sizewherein 90% (by volume) of the particle volume has a smaller particlesize. The d_(as) can be from 0.5 to 15, 0.75 to 15, 1 to 15, 1 to 5, 1to 6, 1 to 8, 5 to 15, 0.5 to 10, 0.5 to 5, and the like. The at leastone amorphous material containing at least one entrapped vaporizablematerial such as water can have a median particle size, d_(a50), of fromabout 0.01 μm to about 100 μm, wherein d_(a50) is a median particle sizewhere 50% (by volume) of the particles of the distribution have asmaller particle size. The median particle size, d_(a50), can be fromabout 1 μm to about 5 μm, from 1 to 5 μm, 1 to 90 μm, 1 to 80 μm, 1 to70 μm, 1 to 60 μm, 1 to 50 μm, 1 to 40 μm, 1 to 30 μm, 1 to 20 μm, 1 to10 μm, 10 to 90 μm, 20 to 80 μm, 30 to 70 μm, and the like, whereind_(a50) is a median particle size where 50% (by volume) of the particlesof the distribution have a smaller particle size.

The ratio for forming the in-situ formed whiskers in the composite canbe from about (20 wt % first metal oxide or precursor/80 wt % secondmetal oxide(s) or metal oxide precursor) to about (60 wt % first metaloxide or precursor/40 wt % second metal oxide(s) or metal oxideprecursor).

The pre-formed whiskers (or pre-formed whisker seeds) can be present inan amount of from 0.01 wt % to 40 wt %, such as from about 2% by weightto about 25% by weight, of the green body material. The inventors haveunexpectedly found that whiskers, such as mullite whiskers, incorporatedinto the green body material acts as whisker formation seeds allowingearly onset of whisker formation and at temperatures near the bottom ofthe range typically required for formation of mullite whiskers. Ifsintering temperature reaches temperatures of about 1500° C., theconversion of alumina or alumina precursor and siliceous material toglass is nearly complete and an effective composite is not formed. Inaddition, higher temperatures tend to form pores in the amorphous phaselowering the strength and toughness of the resulting glass-ceramicmaterial. Mullite whiskers can be naturally occurring in cenospheres andcan be present in an amount of from about 2% by weight to about 40% byweight of the cenospheres. In addition or alternatively, small mullitewhiskers directly formed or ground can be added to the green bodymaterial, for instance, in an amount of from about 0.5% by weight toabout 2% by weight of the green body material.

The whisker promoter can be B₂O₃ and/or one or more transition metaloxides. Examples include, but are not limited to, Fe₂O₃, TiO₂, CoO, NiOor any combinations thereof. The whisker promoter can be used in anyamount, for instance from about 0.1 wt % to 5 wt %, or from about 1% byweight to about 2% by weight of the green body material or mixture. Afurther novel aspect of the present invention is the use of B₂O₃ and/ortransition metal oxide whisker promoters that can include Fe₂O₃, TiO₂,CoO, and/or NiO or any combinations thereof, to control the growth ofwhiskers, such as mullite whiskers. Materials that promote the formationof mullite whiskers are typically compounds that include fluorine. U.S.Pat. No. 4,911,902 mentions the use of SiF₄ in an anhydrous environmentto produce bar-like topaz as a precursor to form mullite whiskers. L. B.Kong, et al (L. B, Kong. “Effect of transition metal oxides on mullitewhisker formation from mechanochemically activated powders,” MaterialScience and Engineering A359 (2003): 75-81, Print.) stated that theaddition of transition metal oxides have shown significant influence onthe mullite formation temperature and the morphology of the mullitewhiskers from the oxide mixtures activated by a high-energy ball millingprocess. The inventors have unexpectedly found that various transitionmetal oxides and combinations thereof produce a balance of mullitewhiskers and amorphous alumina and silica in a glass-ceramic composite.The transition metal oxides in the present invention can include Fe₂O₃,TiO₂, CoO, and/or NiO, or combinations thereof. Furthermore, theinventors have unexpectedly discovered that trace amounts of Fe₂O₃ andother iron oxide compounds present in cenospheres and fly ash caneffectively act as the transition metal oxide promoter.

The inventors have also found that in-situ mullite whiskers in the finalproduct can form when a siliceous material that contains mullite, suchas a cenosphere, is saturated with amorphous silica and alumina. Aftersintering, a phase separation within the amorphous phase of thesiliceous material occurs resulting in an amorphous phase deficient insilica and alumina and a crystalline mullite whisker phase. During phaseseparation, some of the amorphous alumina in the amorphous phase cancrystallize and enhance the strength of the resulting composite.

The inventors have also found that a whisker phase, such as a mullitewhisker phase, can be formed without production of in-situ mullite. Thereactive sintering process effectively can form an amorphous phase and amullite phase from mullite that existed in the starting green bodymaterials.

Whiskers, such as mullite whiskers, can be present as a result of bothreactive sintering and/or whiskers present in the starting green bodymaterial.

For exemplary purposes, the following example is provided. Materialsother than those mentioned below can be used.

The present invention, as one example, relates to a method for producinga glass-ceramic composite. The method includes the steps of forming agreen body. The green body can be formed from a green body material thatincludes:

-   -   i. (a) alumina and/or at least one alumina precursor, and (b) a        siliceous material;    -   ii. at least one amorphous material containing at least one        entrapped vaporizable material such as water, where        -   alumina and silica present in the alumina and/or at least            one alumina precursor, and/or siliceous material and/or the            at least one amorphous material are present in a weight            ratio such that whiskers, such as mullite whiskers, form in            said glass-ceramic composite upon sintering, and    -   iii. pre-formed mullite whiskers (e.g., a minor amount), and    -   iv. at least one whisker promoter, preferably in the absence of        fluorine or fluorine compounds.

The green body is then subjected to sintering under sintering conditionsto form a glass-ceramic composite having at least one whisker phase,such as a mullite whisker phase, and at least one amorphous phase.

The alumina precursor can be or include aluminum hydroxide, bauxite,gibbsite, boehmite or diaspore or any combination thereof. The aluminaor alumina precursor can have any particle size distribution. Forexample, the particle size distribution, d_(as), can be from about 0.5to about 15, wherein, d_(as)={(d_(a90)−d_(a10))/d_(a50)} wherein d_(a10)is a particle size wherein 10% (by volume) of the particles have asmaller particle size, d_(a50) is a median particle size wherein 50% (byvolume) of the particles have a smaller particle size, and d_(a90) is aparticle size wherein 90% (by volume) of the particle volume has asmaller particle size. The d_(as) can be from 0.5 to 15, 0.75 to 15, 1to 15, 1 to 5, 1 to 6, 1 to 8, 5 to 15, 0.5 to 10, 0.5 to 5, and thelike. The alumina or alumina precursor can have a median particle size,d_(a50), of from about 0.01 μm to about 100 μm, wherein d_(a50) is amedian particle size where 50% (by volume) of the particles of thedistribution have a smaller particle size. The median particle size,d_(a50), can be from about 1 μm to about 5 μm, from 1 to 5 μm, 1 to 90μm, 1 to 80 μm, 1 to 70 μm, 1 to 60 μm, 1 to 50 μm, 1 to 40 μm, 1 to 30μm, 1 to 20 μm, 1 to 10 μm, 10 to 90 μm, 20 to 80 μm, 30 to 70 μm, andthe like, wherein d_(a50) is a median particle size where 50% (byvolume) of the particles of the distribution have a smaller particlesize.

The siliceous material is any silicon containing material, such assilicate containing material, silicon containing minerals or ore,silicates, silicon oxides, and the like. The siliceous material can beor include one or more cenospheres, fly ash or any combination thereof.The siliceous material can be natural, synthetic, or a by-product. Thesiliceous material can be or include silicate materials, quartz,feldspar, zeolites, bauxite, calcined clays or any combination thereof.The siliceous material can have any particle size, such as a particlesize distribution, The d_(as) can be from 0.5 to 15, 0.75 to 15, 1 to15, 1 to 5, 1 to 6, 1 to 8, 5 to 15, 0.5 to 10, 0.5 to 5d_(ss), of fromabout 0.5 to about 15, wherein, d_(ss)={(d_(s90)−d_(s10))/d_(s50)}wherein d_(s10) is a particle size wherein 10% (by volume) of theparticles have a smaller particle size, d_(s50) is a median particlesize wherein 50% (by volume) of the particles have a smaller particlesize, and d_(s90) is a particle size wherein 90% (by volume) of theparticle volume has a smaller particle size. The d_(as) can be from 0.5to 15, 0.75 to 15, 1 to 15, 1 to 5, 1 to 6, 1 to 8, 5 to 15, 0.5 to 10,0.5 to 5 and the like. The siliceous material can have a median particlesize, d_(a50), of from about 0.01 μm to about 100 μm, wherein d_(a50) isa median particle size where 50% (by volume) of the particles of thedistribution have a smaller particle size. The median particle size,d_(a50), can be from about 1 μm to about 5 μm, from 1 to 5 μm, 1 to 90μm, 1 to 80 μm, 1 to 70 μm, 1 to 60 μm, 1 to 50 μm, 1 to 40 μm, 1 to 30μm, 1 to 20 μm, 1 to 10 μm, 10 to 90 μm, 20 to 80 μm, 30 to 70 μm, andthe like, wherein d_(a50) is a median particle size where 50% (byvolume) of the particles of the distribution have a smaller particlesize.

The amorphous material and/or other material that can contain at leastone entrapped vaporizable material can be perlite, stober silica, anyamorphous volcanic rock containing silica, alumina, and the like. Theamorphous material can have any particle size and/or any particle sizedistribution. The d_(as) can be from 0.5 to 15, 0.75 to 15, 1 to 15, 1to 5, 1 to 6, 1 to 8, 5 to 15, 0.5 to 10, 0.5 to 5. The d_(ss) can befrom about 0.5 to about 15, wherein, d_(ss)={(d_(a90)−d_(s10))/d_(s50)}wherein d_(s10) is a particle size wherein 10% (by volume) of theparticles have a smaller particle size, d_(s50) is a median particlesize wherein 50% (by volume) of the particles have a smaller particlesize, and d_(s90) is a particle size wherein 90% (by volume) of theparticle volume has a smaller particle size. The d_(as) can be from 0.5to 15, 0.75 to 15, 1 to 15, 1 to 5, 1 to 6, 1 to 8, 5 to 15, 0.5 to 10,0.5 to 5 and the like. The at least one amorphous material containing atleast one entrapped vaporizable material, such as water, can have amedian particle size, d_(a50), of from about 0.01 μm to about 100 μm,wherein d_(a50) is a median particle size where 50% (by volume) of theparticles of the distribution have a smaller particle size. The medianparticle size, d_(a50), can be from about 1 μm to about 5 μm, from 1 to5 μm, 1 to 90 μm, 1 to 80 μm, 1 to 70 μm, 1 to 60 μm, 1 to 50 μm, 1 to40 μm, 1 to 30 μm, 1 to 20 μm, 1 to 10 μm, 10 to 90 μm, 20 to 80 μm, 30to 70 μm, and the like, wherein d_(a50) is a median particle size where50% (by volume) of the particles of the distribution have a smallerparticle size.

As an option, the particle size distribution and/or the median particlesize of the alumina or precursor thereof, the siliceous material, andthe at least one amorphous material containing at least one entrappedvaporizable material can be the same or different, or can be within 1%,5%, 10%, 15%, 20%, 25% of each other.

The ratio for forming in-situ formed mullite whiskers in theglass-ceramic composite can be from about (20 wt % siliceous material/80wt % alumina or alumina precursor) to about (60 wt % siliceousmaterial/40 wt % alumina or alumina precursor), wherein the wt % ofsiliceous material is based on SiO₂ amount and the alumina/aluminaprecursor is based on alumina amount. The ratio is calculated using thetotal weight of siliceous material and alumina present collectively inthe alumina or precursor thereof, the siliceous material, and the atleast one amorphous material containing at least one entrappedvaporizable material.

The pre-formed whiskers can be obtained from cenospheres themselves. Thepre-formed whiskers can be present in an amount of from 0.01 wt % to 40wt %, such as from about 2% by weight to about 25% by weight, of thecenospheres. Thus, if cenospheres are used in part or entirely as thesiliceous material, the cenospheres can serve a dual purpose, namely asthe siliceous source and as the pre-formed whisker source for purposesof the method. The pre-formed mullite whiskers can be present in anamount of from about 0.5% by weight to about 20% by weight of said greenbody material.

The whisker promoter can be one or more transition metal oxides.Examples include, but are not limited to, B₂O₃, Fe₂O₃, TiO₂, CoO, NiO orany combinations thereof. The whisker promoter can be used in anyamount, for instance from about 0.1 wt % to 5 wt %, or from about 1% byweight to about 2% by weight of the green body material or mixture.

In general, for the methods of the present invention, the green bodymaterial can include at least one sintering promoter, such as asintering aid, a glassy phase formation agent, a grain growth inhibitor,a ceramic strengthening agent, a crystallization control agent, or phaseformation control agent, or any combinations thereof. The sinteringpromoter can be or include zirconium, iron, magnesium, alumina, bismuth,lanthanum, silicon, calcium, cerium, yttrium, a silicate, a borate orany combinations thereof. The sintering promoter can be or include acompound containing zirconium, iron, magnesium, alumina, bismuth,lanthanum, silicon, calcium, cerium, yttrium, a silicate, a borate orany combination thereof.

The green body material can include at least one binder. The binder canbe or include a wax, a starch, polyvinyl alcohol, a sodium silicatesolution, or a low molecular weight functionalized polymer (e.g., 1,000MW to 100,000 MW or 500 MW to 5,000 MW) or any combinations thereof. Abinder may be used to facilitate the formation of the green bodymixture.

The green body material can further include at least one dispersant. Thedispersant can be or include at least one surfactant. A dispersant maybe used to facilitate a uniform mixture of alumina or alumina precursorand a siliceous material in the green body material. Specificdispersants can include, but are not limited to, DOLAPIX CE 64(Zschimmer & Schwarz, GmbH), DARVAN C (RT Vanderbilt Company. IndustrialMinerals & Chemicals) and similar materials which may comprise fromabout 0% by weight to about 5% by weight of the green body material orany other amount to assist in the dispersion of materials.

The green body material can include at least one fluxing agent. Thefluxing agent can be or include nepheline syenite, feldspar, syntheticclay, natural clay or any combinations thereof. The fluxing agent can bepresent in an amount of from about 0.1 wt % to about 25 wt %, based onthe weight of the green body material.

The green body material can further include at least one slurryingagent. The slurrying agent can be or include water, an organic solventor any combinations thereof. The slurrying agent can be present in anamount of from about 5 wt % to about 60 wt %, based on solid loading.

The green body can be formed as one material or can be formed as one ormore layers of green body material. Each layer can be the same ordifferent from each other with respect to composition and/or thickness.The thickness of each layer can be any amount, such as from 1 micron to1,000 microns. The thickness can be uniform or non-uniform.

The green body can be produced by spray drying, die pressing, extrusioncoating, fluidized bed coating, mixer granulation, high shear mixing,roller compaction injection molding, tumbling or any combinationthereof.

The green body can further include a template, wherein the green bodymaterial coats, forms a layer(s), or encapsulates the template, such asa solid or hollow template. The template can be or include a cenosphere,a micro glass sphere, a synthetic cenosphere, a polymer bead or anycombinations thereof.

When a template is present and is a cenosphere or other ceramicmaterial, the sintering in the process of the present invention can format least one whisker phase, such as mullite whisker phase, and anamorphous phase in the template.

The green body can be formed by deposition of the green body materialonto a template such as a hollow template. The deposition can beachieved by spray drying, fluidized bed coating or any combinationsthereof. The spray drying can be performed at an air temperature of fromabout 40° C. to about 90° C., an air flow of from about 90 liters perminute to about 150 liters per minute, and/or a nozzle air pressure offrom about 10 psig to about 25 psig.

The sintering can be performed in the presence of a gas. The gas can beor include oxygen, such as from about 100 ppm to about 100% by weightoxygen, or from about 250 ppm to about 90% by weight oxygen, or fromabout 500 ppm to about 79% by weight oxygen, or from about 1000 ppm toabout 50% by weight oxygen.

The sintering can occur in any sintering device (e.g., furnace, oven)such as with induction heating. The sintering is controlled so as topromote reactive or reaction sintering and not solid state sintering.The sintering can occur in a rotary kiln, microwave, tunnel kiln,shutter kiln, electric furnace, gas furnace, convection furnace, rollerhearth, chain hearth, pusher sled, vertical shaft furnace or anycombinations thereof. The sintering can be self-propagation hightemperature sintering, radiation sintering, plasma sintering, sparkplasma sintering and the like.

The sintering can be performed under a pressure of from about 0.1×10⁵ Pato about 10×10⁵ Pa, such as from about 0.5×10⁵ Pa to about 7×10⁵ Pa, orfrom about 1×10⁵ Pa to about 5×10⁵ Pa.

The sintering can be performed at a temperature from about 500° C. toabout 2500° C. The sintering can be performed at an elevated pressure,for instance, at a pressure from about 0.1 MPa to about 200 MPa forabout 1 hour to about 20 hours. The sintering preferably occurs at atemperature below 1400° C., such as from 1100° C. to about 1300° C., forabout 30 minutes to 8 hours, and more preferably from 2 to 6 hours. Thesintering temperatures referred to herein are the temperature of thematerial being sintered. Other sintering temperatures/times can be at atemperature from about 1100° C. to about 1300° C. for about 1 hour toabout 20 hours. Another example of the pressure during sintering is fromabout 0.1 MPa to about 200 MPa.

The sintering can be performed at any firing rate, such as a firing rateof from about 0.01° C./min to about 2000° C./min.

As indicated above, the final product, for instance formed from thismethod or other methods can be composite material, such as aglass-ceramic composite material that is or includes a sintered bodyhaving at least one whisker phase and an amorphous phase and optionally,at least one crystalline phase, such as a crystalline particulate phase.The amorphous phase can be or include at least one ceramic or metaloxide. The amorphous phase can further include unreacted particles, suchas unreacted metal oxide(s). The composite material can further includea template. The template can be a solid or hollow sphere. The hollowsphere can be or include at least one cenosphere, a micro glass sphere,a synthetic cenosphere, a polymer bead or any combination thereof. Thetemplate can have or include at least one whisker phase (e.g., in-situmullite whisker phase) and an amorphous phase. The whiskers in thecomposite can have diameters of from about 0.05 μm to about 2 μm, and/oraspect ratios of from about 10 to about 50, and/or lengths of from about1 μm to about 50 μm.

The phases of the glass-ceramic composite can be or have 3-3connectivity for the whisker phase and the amorphous phase. The phasesof the composite can be or have 3-3-0 connectivity for the whiskerphase, the amorphous phase and the unreacted metal oxide, respectively.The phases can be or have 3-3-0-0 connectivity for the whisker phase,the amorphous phase, and the two or more types of unreacted metal oxidematerial (unreacted first metal oxide and unreacted second metal oxide)respectively.

The amorphous phase can include or be ceramic, and for instance caninclude alumina and/or silica. The amorphous phase can further includeunreacted material (e.g., particles), such as alumina, aluminaprecursor, siliceous material, and/or the amorphous material (e.g.,perlite) or any combination thereof.

Referring to the preferred method and starting ingredients, the finalproduct, for instance, formed from this method or other methods can be aglass-ceramic composite material that is or includes a sintered bodyhaving at least one whisker phase, such as a mullite whisker phase, andan amorphous phase. The amorphous phase can be or include at least oneceramic, such as alumina and/or silica. The amorphous phase can furtherinclude unreacted particles, such as alumina, alumina precursor,siliceous material or any combinations thereof. The composite materialcan further include a template. The template can be a solid or hollowsphere. The hollow sphere can be or include at least one cenosphere, amicro glass sphere, a synthetic cenosphere, a polymer bead or anycombination thereof. The template can have or include at least onewhisker phase, such as a mullite whisker phase, (e.g., in-situ mullitewhisker phase) and an amorphous phase. The whiskers, such as mullitewhiskers, in the glass-ceramic composite can have diameters of fromabout 0.05 μm to about 2 μm, and/or aspect ratios of from about 10 toabout 50, and/or lengths of from about 1 μm to about 50 μm.

The phases of the glass-ceramic composite can be or have 3-3connectivity for the whisker phase (e.g., mullite whisker phase) and theamorphous phase. The phases of the glass-ceramic composite can be orhave 3-3-0 connectivity for the whisker phase (e.g., mullite whiskerphase), the amorphous phase and the unreacted alumina or alumnaprecursor, respectively. The phases of the glass-ceramic composite canbe or have 3-3-0 connectivity for the whisker phase (e.g., mullitewhisker phase), the amorphous phase and the unreacted siliceousmaterial, respectively. The phases of the glass-ceramic composite can beor have 3-3-0-0 connectivity for the whisker phase (e.g., mullitewhisker phase), the amorphous phase, the unreacted siliceous materialand the unreacted alumina or alumna precursor, respectively.

The amorphous phase can include or be ceramic, and for instance caninclude alumina and/or silica. The amorphous phase can further includeunreacted material (e.g., particles), such as alumina, aluminaprecursor, siliceous material, amorphous material (e.g., perlite), orany combination thereof.

As indicated, the composite of the present invention can be considered aproppant or used as a proppant.

The proppant can have at least one of the following characteristics:

-   -   a. an overall diameter of from 80 microns to about 3,000 microns        or 90 microns to 2,000 microns;    -   b. Krumbein sphericity of at least about 0.5 and a roundness of        at least about 0.5;    -   c. a crush strength of about 10 MPa or greater;    -   d. a specific gravity of from about 1.0 to about 3.0;    -   e. a porosity of from about 1% to about 70%;    -   f. at least 90% (by number or by volume) of proppant pores        having a pore size of from about 0.1 μm to about 10 μm,    -   g. at least 80% (by number or by volume) of proppant pores are        not in contact with each other,    -   h. pores that are generally spherical in shape and have a        smooth, glassy interior surface.    -   i. a smooth external surface.        All of a. through i. can be present, or any two, three, four,        five, six, seven, or eight of the properties/characteristics.

The proppants can be used in a method to prop open subterraneanformation fractures and can involve introducing a proppant formulationthat includes one or proppants of the present invention, into thesubterranean formation. The method can be for treating a subterraneanproducing zone penetrated by a well bore, and can include the steps ofpreparing or providing a treating fluid that includes a fluid, energizedfluid, foam, or a gas carrier having the proppant of the presentinvention suspended therein, and pumping the treating fluid into thesubterranean producing zone whereby the particles are deposited therein.The treating fluid can be a fracturing fluid and the proppant particlescan be deposited in the fractures formed in the subterranean producingzone. The treating fluid can be a gravel packing fluid and the particlescan be deposited in the well bore adjacent to the subterranean producingzone.

The present invention further relates to a matrix that includes aplurality of the proppants of the present invention and at least onesolid matrix material in which the proppant is distributed.

The configuration of the glass-ceramic article being formed can takemany shapes including a sphere, elliptical, doughnut shape, rectangularor any shape necessary to fulfill a useful application. In the case of asphere, the sphere can encapsulate a template. The template may be ahollow or solid, and may be a glassy or glass-ceramic sphere, or anorganic sphere. Hollow spheres are typically used as templates inapplications where it is desirable to produce particles with lowspecific gravity. Spheres with an overall diameter from about 90 μm toabout 2000 μm (e.g., 100 μm or higher) are typical for proppants. FIG. 1shows such a proppant with an outer shell and an inner shell. Mechanicalanalysis of ceramic or glass-ceramic spheres including a hollow templateunder load indicates that tensile stress is the major cause of thefracture since the ceramic materials are typically strong in compressivestrength but weak in tensile strength. Because of this, making the innershell strong and tough has been a great challenge. The present inventiontoughens the inner shell by converting the template into a specialtextured microstructure with the toughening agent generated in-situ. Thetoughening agent can be entangled whiskers, such as mullite whiskers,with diameters from about 0.05 μm to about 2 μm, and/or aspect ratiosfrom about 10 to about 50, and/or lengths from about 1 μm to about 50 μmwith the interstitial space filled with glasses (e.g., a glassy phase oramorphous phase), such as alumina and/or silica and/or other particulatematerials. Any unreacted metal oxide particulates, such as aluminaand/or other particulate ceramic material(s), can serve as a tougheningagent.

Furthermore, the at least one amorphous material containing at least oneentrapped vaporizable material, when sintered under controlledconditions, promotes a smooth surface at the template interface, at theouter surface of the proppant, and/or at any location within theproppant. Small, spherical pores are produced by the liberation of wateror other vaporizable material under controlled sintering conditions witha smooth, glassy interior surface. The smooth surfaces as describedabove can contribute to increased strength and toughness in theproppant. The surface roughness for a proppant made from a green bodythat included an amorphous material with an entrapped vaporizablematerial generally has a lower surface roughness by at least 1%, atleast 5%, at least 10%, at least 15%, at least 20%, compared to the sameproppant but without using an amorphous material having an entrappedvaporizable material.

The composition of the outer shell surrounding the template can be sodesigned that the components of the outer shell react with the template(part or all of the template) to convert it in situ into a preferablytough glass-ceramic structure with a microstructure having a mullitewhisker-reinforced composite. Both the outer shell and the template canundergo viscous reaction sintering to produce a glass-ceramic compositewith 3-3, 3-3-0, 3-3-0-0, 3-2, 3-2-0, 3-2-0-0, 3-1, 3-1-0 or 3-1-0-0connectivity in each phase in the structure. A phase connectivity of 3means that the material in that phase is self-connected in threedimensions. A phase connectivity of 2 means that the material in thatphase is self-connected in two dimensions. A phase connectivity of 1means that the material in that phase is self-connected in onedimension. A phase connectivity of 0 means that the material in thatphase is not self-connected. A 3-3-0 connectivity composite is one whereone phase, typically the glass or ceramic phase, is self connected inthree dimensions, a second phase, typically the infiltrate or whiskerphase, is self connected in three dimensions and a third phase,typically particulates or other materials embedded in the glass orceramic phase is not self connected as in the case of discreteparticles. When the concentration of whiskers, such as mullite whiskers,is high, the whiskers can have a 3 connectivity because the whiskers arein close proximity to each other and become entangled in threedimensions. When the concentration of whiskers, such as mullitewhiskers, is relatively low, the whiskers can have a 1 connectivitywhere the whiskers are not in close proximity and tend to exist asdiscrete and separate whiskers aligned in one dimension. A viscous phaseis distributed uniformly around the whiskers and after viscous reactionsintering forms a glassy phase with 3 connectivity where the glassymaterial is self connected in three dimensions. The resulting compositeof a high concentration of whiskers in a glassy matrix has 3-3connectivity. Production of the glassy phase can be accomplished byviscous reaction sintering of a) alumina, aluminum hydroxide, bauxite,gibbsite, boehmite or diaspore or any combination thereof; and b) asiliceous material such as ground cenosheres, fly ash, silica, silicatematerials, quartz, feldspar, zeolites, bauxite, calcined clays or anycombination thereof; and c) the at least one amorphous materialcontaining at least one entrapped vaporizable material such as watersuch as perlite, stober silica, or any combination thereof. There maybe, as an option, unreacted particles of alumina, aluminum hydroxide,bauxite, gibbsite, boehmite or diaspore, ground cenosheres, fly ash,silica, silicate materials, quartz, feldspar, zeolites, bauxite,calcined clays, perlite, pumice, volcanic glass, and/or stober silicaremaining in the glassy phase. These remaining unreacted particles arenot self-connected and have 0 connectivity.

As mentioned above, it can be difficult to disperse whiskers in aviscous green body material. A novel element of this invention is the insitu formation of whiskers in a glassy matrix. As an example, alumina oralumina precursor particles can be combined with a siliceous material(e.g., ground cenospheres) in a weight proportion that favors formationof whiskers, such as mullite whiskers, under conditions of reactivesintering.

The proportion of alumina to silica in the green body mixture of aluminaor alumina precursor, siliceous material preferably ground cenospheresand the at least one amorphous material containing at least oneentrapped vaporizable material such as water, preferably perlite, can beselected to favor the formation of whiskers, such as mullite whiskers,in the glass-ceramic composite matrix. The stoichiometric amount can beabout 28 parts of silica by weight and about 72 parts of alumina byweight, based on the weight of the green body. The ratio may range fromabout 20 parts silica by weight:about 80 parts alumina by weight toabout 60 parts silica by weight:about 40 parts alumina by weight, basedon the weight of the green body.

In the case of spherical glass-ceramic composite particles including ahollow template, the composition of the outer shell preferably has acoefficient of thermal expansion matching that of the template. If theexpansion of the inner and outer shells is significantly different,cracks may form at the interface between the inner and outer shell andstrength of the resulting particle is negatively affected. The reactantsin the outer shell structure react during firing at typically 1200° C.but can be in the range of 1100° C.-1300° C., to form whiskers, such asmullite whiskers, and alumina/silica composites. The whiskers, such asmullite whiskers, can form in the outer shell, the template (innershell), or both. The sintered shell becomes a glass-ceramic composite,such as with 3-3-0 connectivity or 3-1-0 connectivity depending upon theconcentration of whiskers, such as mullite whiskers, formed.

Examination of fractures in glass-ceramic composites with whiskers(e.g., mullite whiskers) show the amorphous phase and the whisker phase.FIG. 2 is an SEM image showing the microstructure of in-situ formedmicrowhiskers on the free and fracture surfaces of a proppant. FIG. 3shows the pull-out effect of the whiskers was also observed on thefracture surface of the proppant sample, indicating the tougheningeffect of the whiskers. Typically, the diametral splitting tensilestrength of the composites mentioned above is over 100 MPa (14500p.s.i.) (e.g., 100 MPa-300 MPa) for an apparent density around 2.5g/cm³.

In one preferred method, a glass-ceramic composite may be produced bythe following general method.

-   -   1. Alumina, cenospheres, and perlite are ground into an        indicated fine particle size and particle size distribution. The        alumina, cenospheres, perlite and any other components, can be        ground independently and blended, or they can be blended and        then co-milled. In either case, the alumina can be homogenously        mixed with and distributed in the cenosphere material and the        perlite or other ceramic materials or ingredients.    -   2. The alumina, cenospheres, perlite, and any other components        and water are added in a predetermined ratio to a high intensity        mixer, and stirred to form a wet homogeneous particulate        mixture. Optionally, a whisker promoter such as Fe₂O₃ may be        added. Suitable commercially available intensive stirring or        mixing devices used for this purpose can have a rotatable        horizontal or inclined circular table and a rotatable impacting        impeller, such as described in U.S. Pat. No. 3,690,622, to        Brunner, the entire disclosure of which is incorporated herein        by reference.    -   3. While the mixture is being stirred, sufficient water can be        added to cause the formation of a composite, that is essentially        spherical pellets of desired size from the mixture of alumina,        cenospheres, perlite and any other components such that intense        mixing action can rapidly disperse the water throughout the        particles. In general, the total quantity of water that is        sufficient to cause essentially spherical pellets to form is        from about 15 to about 30 percent by weight of the mixture of        alumina, cenospheres, perlite and any other components. The        total mixing time can be, for example, from about 2 to about 15        minutes, or other time periods depending on equipment, settings,        compositions, and conditions used. Those of ordinary skill in        the art will understand how to determine a suitable amount of        water to add to the mixer so that substantially round and        spherical pellets are formed.    -   4. Optionally, a minor amount of whiskers, such as mullite        whiskers, may be added to the green body material. The minor        amount of whiskers act as seed whiskers to promote the formation        of the whiskers, such as mullite whiskers, (e.g., at an early        stage) in the viscous reactive sintering process. When        materials, such as ground cenospheres of fly ash, are used as        the siliceous material, a minor amount of whiskers, such as        mullite whiskers, can be naturally present in the cenospheres or        fly ash and supplemental addition of mullite whiskers may be        avoided. The presence of seed mullite whiskers is effective in        production of a whisker phase and a glass-ceramic phase in the        resulting composite.    -   5. Optionally, a binder, for example, various resins or waxes,        starch, or polyvinyl alcohol, may be added to the initial        mixture to improve the formation of pellets and to increase the        green strength of the unsintered pellets. Suitable binders        include, but are not limited to, corn starch, polyvinyl alcohol        or sodium silicate solution, or a blend thereof. Liquid binders        can be added to the mixture and bentonite and/or various resins        or waxes known and available to those of ordinary skill in the        art may also be used as a binder. A suitable binder can be, for        example, CERAFIX K33 (Zschimmer & Schwarz, Inc.—U.S. Division,        Milledgeville, Ga.) or PVA 405 (Kuraray America, Inc., Houston,        Tex.) and similar materials, which may be added at levels of        from about 0 percent by weight to 10% by weight, or from 0.25%        by weight to 1% by weight, or any other amount so as to assist        formation of the pellets. Whether to use more or less binder        than the values reported herein can be determined by one of        ordinary skill in the art through routine experimentation.    -   6. Optionally, a dispersant such as a surfactant may be added to        the initial mixture to improve the homogeneity of the green body        material, improve the dispersion of particulates such as the        metal oxide(s), pore formers such as SiC binder and other        materials and decrease the number of pore former particles that        are in contact with each other. The dispersant also effectively        reduces the time required to make a uniform mixture. Specific        dispersants can include but are not limited to DOLAPIX CE 64        (Zschimmer & Schwarz, GmbH), DARVAN C (RT Vanderbilt Company,        Industrial Minerals & Chemicals) and similar materials which may        be present in an amount of from about 0% by weight to about 5%        by weight of the green body material or any other amount to        assist in the dispersion of materials in the slurrying agent.    -   7. Optionally, a sintering aid may be added to the initial        mixture to enhance the bonding of particles in the ceramic and        speed the sintering process by providing an internal source of        oxygen. Sintering aids can include, but are not limited to,        magnesium oxide (MgO), yttrium oxide (Y₂O₃) and cerium oxides        (CeO₂, Ce₂O₃). Sintering aids may be present in an amount of        from about 0% to about 5% by weight of the green body material        or any other amount to enhance and speed the sintering process.    -   8. Optionally, a fluxing agent may be added to the mixture to        enhance the effect of the perlite to promote smooth surfaces and        interfaces between material phases; to enhance the production of        small, substantially spherical pores with a smooth glassy        interior surface; and/or to enhance formation of a smooth        surface on the outer surface of the proppant. Fluxing agents can        include nepheline syenite, feldspar, synthetic clay, natural        clay, or any combination thereof    -   9. The resulting pellets can be dried and screened to an        appropriate pre-sintering size that can compensate for shrinkage        that occurs during sintering. Rejected oversized and undersized        pellets and powdered material obtained after the drying and        screening steps may be recycled. The pellets may also be        screened either before drying or after firing or both.    -   10. The dried pellets are then fired at a sintering temperature        for a period sufficient to enable recovery of sintered,        spherical pellets having at least one whisker phase, such as a        mullite whisker phase; small, substantially spherical pores with        a smooth glassy interior surface; a smooth, glassy surface on        the outer surface of the proppant; and at least one amorphous        phase meeting predetermined strength specifications. The        sintered pellets can be screened for sizing purposes.

The present invention includes the followingaspects/embodiments/features in any order and/or in any combination:

-   -   1. The present invention relates to a method for producing a        proppant comprising        -   a. forming a green body from a green body material            comprising            -   i. at least one metal oxide or precursor thereof that is                capable of forming whiskers in said proppant and as part                of said proppant,            -   ii. at least one amorphous material containing at least                one entrapped vaporizable material,            -   iii. preformed whiskers, and            -   iv. at least one whisker promoter, optionally in the                absence of fluorine or fluorine compounds;        -   b. reactive sintering said green body under reactive            sintering conditions to form a sintered body comprising            whiskers and at least one amorphous phase.    -   2. The proppant of any preceding or following        embodiment/feature/aspect, wherein said metal oxide comprises        alumina.    -   3. The proppant of any preceding or following        embodiment/feature/aspect, wherein said alumina has a specific        surface area of from 1 to about 50 m²/g.    -   4. The proppant of any preceding or following        embodiment/feature/aspect, wherein said metal oxide comprises        bauxite, alpha, gamma, and/or theta alumina.    -   5. The proppant of any preceding or following        embodiment/feature/aspect, wherein said at least one metal oxide        comprises a first metal oxide and a second metal oxide, wherein        said first metal oxide and said second metal oxide are different        from each other.    -   6. The proppant of any preceding or following        embodiment/feature/aspect, wherein said at least one metal oxide        comprises a first metal oxide and a second metal oxide, wherein        said first metal oxide and said second metal oxide are different        from each other with respect to metal that forms the oxide.    -   7. The proppant of any preceding or following        embodiment/feature/aspect, wherein said amorphous material        comprises perlite, stober silica, pumice, andesite, scoria,        volcanic glasses, or any combination thereof.    -   8. The proppant of any preceding or following        embodiment/feature/aspect, wherein said vaporizable material        comprises water (H₂O) carbon dioxide (CO₂), sulfur dioxide        (SO₂), hydrogen sulfide (H₂S), nitrogen, argon, helium, neon,        methane, carbon monoxide (CO), hydrogen, oxygen, hydrogen        chloride (HCl), hydrogen fluoride (HF), hydrogen bromide (HBr),        nitrogen oxide (NOx), sulfur hexafluoride (SF₆), carbonyl        sulfide (COS), volcanic gases, or any combination thereof    -   9. The proppant of any preceding or following        embodiment/feature/aspect, wherein said amorphous material has a        lower specific gravity than other said green body materials.    -   10. The proppant of any preceding or following        embodiment/feature/aspect, wherein said method further comprises        forming said green body on a template that is porous or        non-porous.    -   11. The proppant of any preceding or following        embodiment/feature/aspect, wherein said method further comprises        forming said green body around a template so as to encapsulate        said template.    -   12. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template is a sphere.    -   13. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template is a hollow        sphere.    -   14. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template is a solid        sphere.    -   15. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template is a        cenosphere.    -   16. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template is an        agglomerated mullite or alumina particles (e.g., particle size        range of from 20 μm to 500 μm, such as 100-200 μm).    -   17. The proppant of any preceding or following        embodiment/feature/aspect, wherein said reactive sintering at        least partially converts said template to a template comprising        in-situ whiskers and at least one amorphous phase.    -   18. The proppant of any preceding or following        embodiment/feature/aspect, wherein concentration of in-situ        whiskers in said template is different from concentration of        whiskers in said sintered body that is on said template.    -   19. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers comprise        mineral or metal oxide whiskers.    -   20. A proppant comprising a sintered body, wherein said sintered        body comprises whiskers and at least one amorphous phase.    -   21. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers comprise        in-situ whiskers.    -   22. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers are present as        a whisker phase that is a continuous phase and optionally, said        proppant further comprising at least one crystalline particulate        phase.    -   23. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers are present as        a whisker phase that is a non-continuous phase.    -   24. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers are uniformly        distributed throughout said sintered body.    -   25. The proppant of any preceding or following        embodiment/feature/aspect, wherein whiskers are present in said        sintered body in a three-dimensional non-woven structure.    -   26. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers have a phase        connectivity of 3.    -   27. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers have a phase        connectivity of 2.    -   28. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers are metal oxide        or mineral derived whiskers.    -   29. The proppant of any preceding or following        embodiment/feature/aspect, further comprising a template.    -   30. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template is a sphere.    -   31. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template is a hollow        sphere.    -   32. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template is a        cenosphere.    -   33. The proppant of any preceding or following        embodiment/feature/aspect, wherein said sintered body        encapsulates said template.    -   34. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template comprises        whiskers and at least one amorphous phase.    -   35. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template comprises        whiskers and at least one amorphous phase wherein concentration        of whiskers in said template is different from concentration of        whiskers in said sintered body that is on said template.    -   36. The proppant of any preceding or following        embodiment/feature/aspect, wherein said template comprise        whiskers and at least one amorphous phase wherein concentration        of whiskers in said template is lower than concentration of        whiskers in said sintered body that is on said template.    -   37. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers in said        template and in said sintered body comprise mineral or metal        oxide derived whiskers.    -   38. The proppant of any preceding or following        embodiment/feature/aspect, wherein said proppant has at least        one of the following characteristics:        -   a. an overall diameter of from about 90 microns to about            2,000 microns;        -   b. a Krumbein sphericity of at least about 0.5 and a            roundness of at least about 0.5;        -   c. a crush strength of about 10 MPa or greater;        -   d. a specific gravity of from about 1.0 to about 3.0;        -   e. a porosity of from about 6% to about 40%;        -   f. at least 90% of proppant pores having a pore size of from            about 0.1 μm to about 10 μm, and        -   g. at least 80% of proppant pores are not in contact with            each other.    -   39. A method to prop open subterranean formation fractures        comprising introducing a proppant formulation comprising the        proppant of the preceding embodiment/feature/aspect 14 into a        subterranean formation.    -   40. A method of treating a subterranean producing zone        penetrated by a well bore comprising the steps of:        -   a. preparing or providing a treating fluid that comprises a            fluid, energized fluid, foam, or a gas carrier having the            proppant of the preceding embodiment/feature/aspect 14            suspended therein, and        -   b. pumping said treating fluid into said subterranean            producing zone whereby said particles are deposited therein.    -   41. The method of any preceding or following        embodiment/feature/aspect, wherein said treating fluid is a        fracturing fluid and said particles are deposited in fractures        formed in said subterranean producing zone.    -   42. The method of any preceding or following        embodiment/feature/aspect, wherein said treating fluid is a        gravel packing fluid and said particles are deposited in said        well bore adjacent to said subterranean producing zone.    -   43. A method for producing a glass-ceramic composite comprising        -   a. forming a green body from a green body material            comprising            -   i. alumina and/or at least one alumina precursor and a                siliceous material in a ratio to form whiskers in said                glass-ceramic composite, and            -   ii. at least one amorphous material containing at least                one entrapped vaporizable material, and            -   iii. whiskers, and            -   iv. at least one whisker promoter in the absence of                fluorine or fluorine compounds;        -   b. sintering said green body under sintering conditions to            form in situ said glass-ceramic composite comprising at            least one mullite whisker phase and at least one amorphous            phase.    -   44. The method of any preceding or following        embodiment/feature/aspect, wherein said whiskers in i,        and/or iii. are mullite whiskers.    -   45. The method of any preceding or following        embodiment/feature/aspect, wherein said alumina precursor        comprises aluminum hydroxide, bauxite, gibbsite, boehmite or        diaspore or any combination thereof.    -   46. The method of any preceding or following        embodiment/feature/aspect, wherein said alumina or alumina        precursor has a particle size distribution, d_(as), from about        0.5 to about 15, wherein, d_(as)={(d_(a90)−d_(a10))/d_(a50)}        wherein d_(a10) is a particle size wherein 10% of the particles        have a smaller particle size, d_(a50) is a median particle size        wherein 50% of the particles have a smaller particle size, and        d_(a90) is a particle size wherein 90% of the particle volume        has a smaller particle size.    -   47. The method of any preceding or following        embodiment/feature/aspect, wherein said alumina or alumina        precursor has a particle size distribution, d_(as), from about        1.0 to about 6.0.    -   48. The method of any preceding or following        embodiment/feature/aspect, wherein the median particle size,        d_(a50), of said alumina or alumina precursor is from about 0.01        μm to about 100 μm, wherein d_(a50) is a median particle size        where 50% of the particles of the distribution have a smaller        particle size.    -   49. The method of any preceding or following        embodiment/feature/aspect, wherein the median particle size,        d_(a50), of said alumina or alumina precursor is from about 1 μm        to about 5 μm, wherein d_(a50) is a median particle size where        50% of the particles of the distribution have a smaller particle        size.    -   50. The method of any preceding or following        embodiment/feature/aspect, wherein said siliceous material        comprises cenospheres, fly ash or any combination thereof.    -   51. The method of any preceding or following        embodiment/feature/aspect, wherein said siliceous material        comprises silicate materials, quartz, feldspar, zeolites,        bauxite, calcined clays or any combination thereof.    -   52. The method of any preceding or following        embodiment/feature/aspect, wherein said siliceous material has a        particle size distribution, d_(ss), from about 0.5 to about 15,        wherein, d_(ss)={(d_(s90)−d_(s10))/d_(s50)} wherein d_(s10) is a        particle size wherein 10% of the particles have a smaller        particle size, d_(s50) is a median particle size wherein 50% of        the particles have a smaller particle size, and d_(s90) is a        particle size wherein 90% of the particle volume has a smaller        particle size.    -   53. The method of any preceding or following        embodiment/feature/aspect, wherein said siliceous material has a        particle size distribution, d_(ss), from about 1.0 to about 6.0.    -   54. The method of any preceding or following        embodiment/feature/aspect, wherein the median particle size,        d_(s50), of said siliceous material is from about 0.01 μm to        about 100 μm, wherein d_(s50) is a median particle size where        50% of the particles of the distribution have a smaller particle        size.    -   55. The method of any preceding or following        embodiment/feature/aspect, wherein the median particle size,        d_(s50), of said siliceous material is from about 1 μm to about        5 μm, wherein d_(s50) is a median particle size where 50% of the        particles of the distribution have a smaller particle size.    -   56. The method of any preceding or following        embodiment/feature/aspect, wherein said amorphous material        containing at least one entrapped vaporizable material comprises        perlite, a stober silica, pumice, andesite, scoria, volcanic        glasses or any combination thereof.    -   57. The method of any preceding or following        embodiment/feature/aspect, wherein said vaporizable material        comprises water (H₂O), carbon dioxide (CO₂), sulfur dioxide        (SO₂), hydrogen sulfide (H₂S), nitrogen, argon, helium, neon,        methane, carbon monoxide (CO), hydrogen, oxygen, hydrogen        chloride (HCl), hydrogen fluoride (HF), hydrogen bromide (HBr),        nitrogen oxide (NOx), sulfur hexafluoride (SF₆), carbonyl        sulfide (COS), volcanic gases, or any combination thereof.    -   58. The method of any preceding or following        embodiment/feature/aspect, wherein said amorphous material        containing at least one entrapped vaporizable material has a        particle size distribution, d_(as), from about 1.0 to about 6.0.    -   59. The method of any preceding or following        embodiment/feature/aspect, wherein the median particle size,        d_(a50), of said amorphous material containing at least one        entrapped vaporizable material is from about 0.01 μm to about        100 μm, wherein d_(a50) is a median particle size where 50% of        the particles of the distribution have a smaller particle size.    -   60. The method of any preceding or following        embodiment/feature/aspect, wherein the median particle size,        d_(a50), of said amorphous material containing at least one        entrapped vaporizable material is from about 1 μm to about 5 μm,        wherein d_(a50) is a median particle size where 50% of the        particles of the distribution have a smaller particle size.    -   61. The method of any preceding or following        embodiment/feature/aspect, wherein said ratio to form whiskers        in said glass-ceramic composite is from about 20% SiO₂        material/80% Al₂O₃ or alumina precursor by weight to about 60%        siliceous material/40% alumina or alumina precursor by weight.    -   62. The method of any preceding or following        embodiment/feature/aspect, wherein said mullite whiskers are        naturally occurring in cenospheres and comprise from about 2% by        weight to about 40% by weight of the cenospheres, wherein said        siliceous material comprises cenospheres, and wherein said        cenospheres contain said mullite whiskers.    -   63. The method of any preceding or following        embodiment/feature/aspect, wherein said mullite whiskers        comprises from about 0.5% by weight to about 20% by weight of        said green body material.    -   64. The method of any preceding or following        embodiment/feature/aspect, wherein said whisker promoter        comprises B₂O₃ and/or at least one transition metal oxide.    -   65. The method of any preceding or following        embodiment/feature/aspect, wherein said transition metal oxide        comprises Fe₂O₃, TiO₂, CoO, NiO, or any combination thereof    -   66. The method of any preceding or following        embodiment/feature/aspect, wherein said whisker promoter        comprises from about 1% by weight to about 2% by weight of said        green body mixture.    -   67. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering is performed        in the presence of a gas.    -   68. The method of any preceding or following        embodiment/feature/aspect, wherein said gas comprises from about        100 ppm to about 100% by weight oxygen.    -   69. The method of any preceding or following        embodiment/feature/aspect, wherein said gas comprises from about        250 ppm to about 90% by weight oxygen.    -   70. The method of any preceding or following        embodiment/feature/aspect, wherein said gas comprises from about        500 ppm to about 79% by weight oxygen.    -   71. The method of any preceding or following        embodiment/feature/aspect, wherein said gas comprises from about        1000 ppm to about 50% by weight oxygen.    -   72. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering comprises        induction heating, rotary kiln, microwave, tunnel kiln, shutter        kiln, electric furnace, gas furnace, convection furnace,        self-propagation high temperature sintering, radiation, plasma,        spark plasma, roller hearth, chain hearth, pusher sled, vertical        shaft furnace or any combination thereof.    -   73. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering is performed        under a pressure of from about 0.1×10⁵ Pa to about 10×10⁵ Pa.    -   74. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering is performed        under a pressure of from about 0.5×10⁵ Pa to about 7×10⁵ Pa.    -   75. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering is performed        under a pressure of from about 1×10⁵ Pa to about 5×10⁵ Pa.    -   76. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering is performed        at a temperature from about 500° C. to about 2500° C., and said        pressure is from about 0.1 MPa to about 200 MPa for about 1 hour        to about 20 hours.    -   77. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering is performed        at a temperature from about 1000° C. to about 1400° C., and said        pressure is from about 0.1 MPa to about 200 MPa for about one        half hour to about 20 hours.    -   78. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering is performed        at a firing rate from about 0.01° C./min to about 2000° C./min.    -   79. The method of any preceding or following        embodiment/feature/aspect, wherein the green body material        further comprises at least one sintering promoter comprising a        sintering aid, a glassy phase formation agent, a grain growth        inhibitor, a ceramic strengthening agent, a crystallization        control agent, or phase formation control agent, or any        combination thereof.    -   80. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering promoter        comprises sodium, potassium, cesium (all alkalines) zirconium,        iron, magnesium, alumina, bismuth, lanthanum, silicon, calcium,        cerium, yttrium, a silicate, a borate or any combination        thereof.    -   81. The method of any preceding or following        embodiment/feature/aspect, wherein said sintering promoter        comprises a compound containing (alkaline compounds, please        expand) zirconium, iron, magnesium, alumina, bismuth, lanthanum,        silicon, calcium, cerium, yttrium, a silicate, a borate or any        combination thereof.    -   82. The method of any preceding or following        embodiment/feature/aspect, wherein said green body material        further comprises a binder.    -   83. The method of any preceding or following        embodiment/feature/aspect, wherein said binder comprises a wax,        a starch, polyvinyl alcohol, a sodium silicate solution, a low        molecular weight functionalized polymer or any combination        thereof.    -   84. The method of any preceding or following        embodiment/feature/aspect, wherein said green body material        further comprises a dispersant.    -   85. The method of any preceding or following        embodiment/feature/aspect, wherein said dispersant comprises a        surfactant.    -   86. The method of any preceding or following        embodiment/feature/aspect, wherein said green body material        further comprises at least one slurrying agent.    -   87. The method of any preceding or following        embodiment/feature/aspect, wherein said slurrying agent        comprises water, an organic solvent or any combination thereof    -   88. The method of any preceding or following        embodiment/feature/aspect, wherein said green body material        further comprises at least one fluxing agent.    -   89. The method of any preceding or following        embodiment/feature/aspect, wherein said fluxing agent comprises        nepheline syenite, feldspar, synthetic clay, natural clay or any        combination thereof.    -   90. The method of any preceding or following        embodiment/feature/aspect, wherein said green body comprises at        least one or more layers of said green body material.    -   91. The method of any preceding or following        embodiment/feature/aspect, wherein said layers are of differing        compositions of said green body material.    -   92. The method of any preceding or following        embodiment/feature/aspect, wherein said mullite whiskers in said        glass-ceramic composite have diameters from about 0.05 μm to        about 2 aspect ratios from about 10 to about 50, and lengths        from about 1 μm to about 50    -   93. The method of any preceding or following        embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3 connectivity for the        mullite whisker phase and the amorphous phase.    -   94. The method of any preceding or following        embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3-0 connectivity for the        mullite whisker phase, the amorphous phase and the unreacted        alumina or alumna precursor.    -   95. The method of any preceding or following        embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3-0 connectivity for the        mullite whisker phase, the amorphous phase and the unreacted        siliceous material.    -   96. The method of any preceding or following        embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3-0 connectivity for the        mullite whisker phase, the amorphous phase and the amorphous        material containing at least one entrapped vaporizable material.    -   97. The method of any preceding or following        embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3-0-0-0 connectivity for the        mullite whisker phase, the amorphous phase, the unreacted        siliceous material, the unreacted alumina or alumna precursor,        and the amorphous material containing at least one entrapped        vaporizable material.    -   98. The method of any preceding or following        embodiment/feature/aspect, wherein said amorphous phase consists        of at least one ceramic comprising alumina, silica, the        amorphous material containing at least one entrapped vaporizable        material or any combination thereof    -   99. The method of any preceding or following        embodiment/feature/aspect, wherein said amorphous phase further        comprises unreacted particles of alumina, alumina precursor,        siliceous material, amorphous material containing at least one        entrapped vaporizable material or any combination thereof    -   100. The method of any preceding or following        embodiment/feature/aspect, wherein said forming a green body is        produced by spray coating, die pressing, extrusion coating,        fluidized bed coating, mixer granulation, high shear mixing,        roller compaction injection molding, tumbling or any combination        thereof.    -   101. The method of any preceding or following        embodiment/feature/aspect, wherein said green body further        comprises a hollow template.    -   102. The method of any preceding or following        embodiment/feature/aspect, wherein said hollow template        comprises a cenosphere, a micro glass sphere, a synthetic        cenosphere, a polymer bead or any combination thereof    -   103. The method of any preceding or following        embodiment/feature/aspect, wherein said green body further        comprises a hollow template and said sintering forms at least        one whisker phase and an amorphous phase in said template.    -   104. The method of any preceding or following        embodiment/feature/aspect, wherein said green body is formed by        deposition of said green body material onto said hollow        template.    -   105. The method of any preceding or following        embodiment/feature/aspect, wherein said deposition comprises,        fluidized bed spray coating or any combination thereof.    -   106. The method of any preceding or following        embodiment/feature/aspect, wherein said spray coating is        performed at an air temperature from about 40° C. to about 90°        C., air flow from about 90 liters per minute to about 150 liters        per minute, and nozzle air pressure from about 10 psig to about        25 psig.    -   107. A glass-ceramic composite material comprising a sintered        body having at least one whisker phase and an amorphous phase        and optionally, at least one crystalline particulate phase.    -   108. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein said whisker phase        is a mullite whisker phase.    -   109. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein said amorphous        phase is a ceramics comprising alumina, silica and any        combination thereof.    -   110. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein said amorphous        phase further comprises unreacted particles of alumina, alumina        precursor, siliceous material, amorphous material containing at        least one entrapped vaporizable material or any combination        thereof.    -   111. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, further comprising a        template.    -   112. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein said template is a        hollow sphere comprising a cenosphere, a micro glass sphere, a        synthetic cenosphere, a polymer bead or any combination thereof.    -   113. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein said template is a        solid sphere.    -   114. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein said template        comprises at least one whisker phase and an amorphous phase.    -   115. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein said whiskers in        said glass-ceramic composite have diameters from about 0.05 μm        to about 2 μm, aspect ratios from about 10 to about 50, and        lengths from about 1 μm to about 50 μm.    -   116. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3 connectivity for the        whisker phase and the amorphous phase.    -   117. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3-0 connectivity for the        whisker phase, the amorphous phase and the unreacted alumina or        alumna precursor.    -   118. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3-0 connectivity among the        whisker phase, the amorphous phase and the unreacted alumina or        alumna precursor.    -   119. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3-0 connectivity among the        whisker phase, the amorphous phase and the amorphous material        containing at least one entrapped vaporizable material.    -   120. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein the phases of said        glass-ceramic composite comprises 3-3-0-0-0 connectivity for the        whisker phase, the amorphous phase, the unreacted alumina        material, the unreacted siliceous material, and the amorphous        material containing at least one entrapped vaporizable material.    -   121. The glass-ceramic composite material of any preceding or        following embodiment/feature/aspect, wherein said composite has        at least one of the following characteristics:        -   a. an overall diameter of from about 90 microns to about            2,000 microns;        -   b. a Krumbein sphericity of at least about 0.5 and a            roundness of at least about 0.5;        -   c. a crush strength of about 10 MPa or greater;        -   d. a specific gravity of from about 1.0 to about 3.0;        -   e. a porosity of from about 6% to about 40%;        -   f. at least 90% of proppant pores having a pore size of from            about 0.1 μm to about 10 μm.        -   g. at least 80% of proppant pores are not in contact with            each other.    -   122. A method to prop open subterranean formation fractures        comprising introducing a proppant formulation comprising the        proppant of the preceding embodiment/feature/aspect 89 into a        subterranean formation.    -   123. A method of treating a subterranean producing zone        penetrated by a well bore comprising the steps of:        -   a. preparing or providing a treating fluid that comprises a            fluid, energized fluid, foam, or a gas carrier having the            proppant of the preceding embodiment/feature/aspect 89            suspended therein, and        -   b. pumping said treating fluid into said subterranean            producing zone whereby said particles are deposited therein.    -   124. The method of any preceding or following        embodiment/feature/aspect, wherein said treating fluid is a        fracturing fluid and said particles are deposited in fractures        formed in said subterranean producing zone.    -   125. The method of any preceding or following        embodiment/feature/aspect, wherein said treating fluid is a        gravel packing fluid and said particles are deposited in said        well bore adjacent to said subterranean producing zone.    -   126. A matrix comprising a plurality of the proppant of the        preceding embodiments/feature/aspect 107 and at least one solid        matrix material in which the proppant is distributed.    -   127. A method for producing a proppant comprising        -   a. forming a green body from a green body material            comprising            -   i. at least one metal oxide or precursor thereof that is                capable of forming whiskers in said proppant and as part                of said proppant, and            -   ii. at least one amorphous material containing at least                one entrapped vaporizable material, and            -   iii. optionally preformed whiskers, and            -   iv. at least one whisker promoter, optionally in the                absence of fluorine or fluorine compounds; and            -   v. at least one carbide or metal carbide,        -   b. reactive sintering said green body under reactive            sintering conditions to form a sintered body comprising            in-situ whiskers and at least one amorphous phase.    -   128. The method of any preceding or following        embodiment/feature/aspect, wherein said carbide is SiC.    -   129. A proppant comprising a sintered body, wherein said        sintered body comprises whiskers, at least one glassy phase, and        at least one amorphous phase.    -   130. The proppant of any preceding or following        embodiment/feature/aspect, wherein said sintered body further        comprises at least one carbide or metal carbide.    -   131. The proppant of any preceding or following        embodiment/feature/aspect, wherein said at least one carbide is        SiC.    -   132. The proppant of any preceding or following        embodiment/feature/aspect, wherein said at least one carbide or        metal carbide is present in an amount of from 1% by weight to        25% by weight, based on the weight of the proppant.    -   133. The proppant of any preceding or following        embodiment/feature/aspect, further comprising at least one        carbide or metal carbide in particulate form, and at least one        crystalline particulate phase.    -   134. The proppant of any preceding or following        embodiment/feature/aspect, wherein said at least one crystalline        particulate phase is alumina.    -   135. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers are present as        an in-situ whisker phase that is a continuous phase and        optionally, at least one crystalline particulate phase.    -   136. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers are present as        an in-situ whisker phase that is a non-continuous phase.    -   137. The proppant of any preceding or following        embodiment/feature/aspect, wherein said whiskers are uniformly        distributed throughout said sintered body.    -   138. The proppant of any preceding or following        embodiment/feature/aspect, wherein in-situ whiskers are present        in said sintered body in a three-dimensional non-woven        structure.    -   139. A proppant comprising a sintered sphere having a Krumbein        sphericity of at least about 0.5 and a roundness of at least        about 0.4, and wherein said sphere comprises a) a plurality of        ceramic whiskers or oxides thereof and b) a glassy phase and c)        optionally at least one non-whisker crystalline phase and d)        optionally a plurality of microspheres, wherein said sintered        sphere has a diameter of from about 90 microns to 2,500 microns,        and said sintered sphere has a specific gravity of from 0.8 g/cc        to about 3.8 g/cc, and said proppant has a crush strength of        from about 1,000 psi or greater, and wherein said proppant        includes one or more of the following characteristics:        -   1) said glassy phase is present in an amount of at least 10%            by weight, based on the weight of the proppant;        -   2) said ceramic whiskers have an average length of less than            3.2 microns;        -   3) said ceramic whisker have an average width of less than            0.35 micron;        -   4) said ceramic whiskers have a whisker length distribution,            d_(as), of about 8 or less, wherein,            d_(as){(d_(a90)−d_(a10))/d_(a50)} wherein d_(a10) is a            whisker length wherein 10% of the whiskers have a smaller            length, d_(a50) is a median whisker length wherein 50% of            the whiskers have a smaller whisker length, and d_(a90) is a            whisker length wherein 90% of the whiskers have a smaller            whisker length;        -   5) said proppant having a specific gravity of from 1.6 to            1.8 with a crush strength of at least 2000 psi;        -   6) said proppant having a specific gravity of from 1.8 to 2            with a crush strength of at least 3000 psi;        -   7) said proppant having a specific gravity of from 2 to 2.1            with a crush strength of at least 5,000 psi;        -   8) said proppant having a specific gravity of from 2.25 to            2.35 with a crush strength of at least 8,000 psi;        -   9) said proppant having a specific gravity of from 2.5 to            3.2 with a crush strength of at least 12,000 psi;        -   10) said proppant having a specific gravity of from 2.5 to            3.2 with a crush strength of at least 18,000 psi;        -   11) said proppant having a combined clay amount and            cristobalite amount of less than 20% by weight of proppant;        -   12) said proppant having an free alpha-alumina content of at            least 5% by weight of said proppant;        -   13) said proppant having an HF etching weight loss of less            than 35% by weight of said proppant;        -   14) said proppant having said microspheres present as hollow            glass microspheres having a particle size distribution,            d_(as), of from about 0.5 to about 2.7, wherein,            d_(as)={(d_(a90)−d_(a10))/d_(a50)} wherein d_(a10) is a            particle size wherein 10% of the particles have a smaller            particle size, d_(a50) is a median particle size wherein 50%            of the particles have a smaller particle size, and d_(a90)            is a particle size wherein 90% of the particle volume has a            smaller particle size;        -   15) said proppant having microspheres present wherein said            microspheres are uniformly present in said proppant or in a            layered region of said proppant;        -   16) said ceramic whiskers are present in an amount of from            5% to 60% by weight of said proppant.        -   17) said proppant has a combined clay amount and            cristobalite amount of less than 20% by weight of proppant            and said mullite whiskers are present in an amount of 60% or            more by weight of said proppant;        -   18) said proppant has a high whisker distribution density            based on individual whiskers present in the proppant (# of            whiskers per mg of proppant);        -   19) said proppant has a unimodal whisker distribution;        -   20) said proppant has at least two layers that form a            laminate structure;        -   21) said proppant has at least a first layer and a second            layer that form a laminate structure wherein the average            length of said whiskers in said first layer compared to said            second layer is different;        -   22) said proppant has at least a first layer and a second            layer that form a laminate structure wherein the average            width of said whiskers in said first layer compared to said            second layer is different;        -   23) said whiskers in said proppant are less euhedral and            more anhedral;        -   24) said proppant has at least one radial region of first            whiskers and at least one region of second whiskers, wherein            the average whisker length is different by at least 10%;        -   25) said proppant has at least one radial region of first            whiskers and at least one region of second whiskers, wherein            the average whisker width is different by at least 10%;        -   26) said proppant has a major phase of whiskers of less than            one micron and a secondary minor phase of whiskers of one            micron or higher; and/or        -   27) said ceramic whiskers have a whisker length distribution            having d_(a90), which is a whisker length wherein 90% of the            whiskers have a smaller whisker length, of less than 12            microns.    -   140. The proppant of any preceding or following        embodiment/feature/aspect, wherein one or more of said        characteristics provide stress reducing properties on said        proppant compared to the same proppant but without said        characteristics.    -   141. The proppant of any preceding or following        embodiment/feature/aspect, wherein said proppant has an alumina        content of at least 35% by weight of said proppant.    -   142. The proppant of any preceding or following        embodiment/feature/aspect, wherein said mullite whiskers are        present in an amount of from 10% to 40% by weight of said        proppant.    -   143. The proppant of any preceding or following        embodiment/feature/aspect, further comprising quartz.    -   144. The proppant of any preceding or following        embodiment/feature/aspect, further comprising quartz in an        amount of from 0.1 wt % to 1 wt % based on the weight of the        proppant.    -   145. The proppant of any preceding or following        embodiment/feature/aspect, wherein said proppant comprises at        least one layered shell encapsulating a hollow spherical        template.    -   146. The proppant of any preceding or following        embodiment/feature/aspect, wherein said proppant comprises at        least one layered shell encapsulating a hollow spherical        template, and said nano-microspheres are present in said at        least layered shell.

The present invention can include any combination of these variousfeatures or embodiments above and/or below as set forth in sentencesand/or paragraphs. Any combination of disclosed features herein isconsidered part of the present invention and no limitation is intendedwith respect to combinable features.

The present invention will be further clarified by the followingexamples, which are intended to be exemplary of the present invention.

EXAMPLES Example 1

Raw, unexpanded perlite powder (Grade #270, Hess Pumice Products, 100Hess Dr. Malad City, Idaho 83252) with initial particle size of 595 μm(−30 mesh) was attrition-milled to reach on average particle size ofaround 0.5 μm and then mixed with cenospheres (Grade Sphere One SGTemperate, 8020 Tyler Blvd, Suite #100, Kish Company Inc. Mentor, Ohio44060) and alumina (Grade AC-300 Ungrounded, Aluchem, 14782 Beaver Pike,Jackson, Ohio 45640) in deionized water. The mixture was ball-milledwith high purity alumina media for 4 hours. Before blending with theperlite, the cenosphere and alumina powders were also attrition-milleddown to an average particle size of around 0.5 μm. The mixed slurry wasthen dried in an oven at 125° C. for 4-8 hours and the dry powder wassieved through a 120 mesh screen. Pellets of approximately 0.5″×0.2″were uniaxially pressed at 12 MPa and sintered at 1175° C.-1250° C. from2 hours to 6 hours in air. After the firing, the pellets were cleaned.

Split tensile strength was determined by ASTM C 1144-89. “Standard TestMethod for Splitting Tensile Strength for Brittle Nuclear Waste Forms.”From FIG. 5, it can be seen that a significant increase in strength wasachieved as the initial concentration of perlite was increased. Only aslight increase in specific gravity was observed with increasing perliteconcentration. Maximum crush strength was achieved near a composition of50 wt % Al₂O₃, 30 wt % Cenoshperes, and 20 wt % Perlite. Reduction incrush strength occurs when the concentration of perlite was increased ordecreased from approximately 20 wt %. The possible reasons forimprovement in strength may involve (1) reduction in the numbers ofpores and sizes of pores without too much increase in specific gravity;(2) smooth pore surfaces and outer surface of the proppant produced bythe fused perlite leading to release of stress concentrations at thesurface of the pores and the surface of the proppant; and/or (3)optimization in phase constitution (optimization in phase percentagesamong mullite, alumina and amorphous phases).

X-ray diffraction (phase analysis) was conducted using X-raydiffractometer Siemens D5000. Microstructure. Observations wereperformed using VEGA-II Scanning Electron Microscopes (Tescan USA Inc.508 Thomson Park Drive, Cranberry Twp, Pa. 16066). FIG. 6 shows theX-ray diffraction pattern from a sintered pellet where the green bodycomposition was approximately 50 wt % Al₂O₃, 30 wt % Cenospheres, 20 wt% Perlite. Corundum (α-Alumina), mullite, and the amorphous material arethe major phases indentified in the sintered pellet. The materialcomprising the wall of a cenosphere had a specific gravity of about2.65. Unexpanded perlite has a specific gravity of about 2.2. Theinclusion of proportionately more perlite relative to cenospheres willreduce the average specific gravity of the proppant. The inclusion ofproportionately more perlite relative to cenospheres reduces the numberand size of pores produced which tends to increase the average specificgravity of the proppant. The net effect is relatively little change inaverage specific gravity with an accompanying significant increase incrush strength. This is an unexpected and useful result, improving thebalance of crush strength and specific gravity in the proppant. FIG. 7shows a comparison of various alumina, silica, perlite compositesincluding formulations with no perlite. The compositions containingperlite show an improved balance of specific gravity and crush strength.In FIG. 7, A2, A14, A16SG, A1000, AC2, and AKP50 are grade names ofalumina from different vendors. SiC, B4C and PMMA are additives to thestandard alumina-cenosphere composition.

FIG. 8 is an image produced from a scanning electron microscope atmagnification 5.0 kx showing pores in a proppant produced with thecomposition: 50 wt % Al₂O₃, 30 wt % Cenospheres, 20 wt % Perlite. FIG. 9is an image produced from a scanning electron microscope atmagnification 10.0 kx showing pores in a sintered pellet, where thegreen body composition was approximately 50 wt % Al₂O₃, 30 wt %Cenospheres. The pores in FIG. 8 (50 wt % Al₂O₃, 30 wt % Cenospheres,and 20 wt % Perlite) are smaller in number and size when compared to thepores in FIG. 9 (50 wt % Al₂O₃ and 30 wt % Cenospheres), demonstratingone of the benefits of incorporating perlite in the proppantformulation.

Applicants specifically incorporate the entire contents of all citedreferences in this disclosure. Further, when an amount, concentration,or other value or parameter is given as either a range, preferred range,or a list of upper preferable values and lower preferable values, thisis to be understood as specifically disclosing all ranges formed fromany pair of any upper range limit or preferred value and any lower rangelimit or preferred value, regardless of whether ranges are separatelydisclosed. Where a range of numerical values is recited herein, unlessotherwise stated, the range is intended to include the endpointsthereof, and all integers and fractions within the range. It is notintended that the scope of the invention be limited to the specificvalues recited when defining a range.

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the present specification andpractice of the present invention disclosed herein. It is intended thatthe present specification and examples be considered as exemplary onlywith a true scope and spirit of the invention being indicated by thefollowing claims and equivalents thereof.

What is claimed is:
 1. A method for producing a proppant comprising a.forming a green body from a green body material comprising i. at leastone metal oxide or precursor thereof that is capable of forming whiskersin said proppant and as part of said proppant, wherein a microstructureof the whiskers includes anisotropic crystals elongated along a C-axis,ii. at least one amorphous material containing at least one entrappedvaporizable material, iii. preformed whiskers, wherein the preformedwhiskers are present in the green body material in an amount less than 2wt % based on a weight, wherein the preformed whiskers are present as anon-continuous phase of the green body material; iv. at least onewhisker promoter, optionally in the absence of fluorine or fluorinecompounds; and v. a fluxing agent: b. reactive sintering said green bodyunder reactive sintering conditions to form a sintered body comprisingwhiskers and at least one amorphous phase, wherein the whiskers arenon-uniformly distributed.
 2. The method of claim 1, wherein said atleast one metal oxide comprises a first metal oxide and a second metaloxide, wherein said first metal oxide and said second metal oxide aredifferent from each other.
 3. The method of claim 1, wherein saidamorphous material comprises perlite, stober silica, pumice, andesite,scoria, volcanic glasses, or any combination thereof.
 4. The method ofclaim 3, wherein said vaporizable material comprises water (H₂O) carbondioxide (CO₂), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), nitrogen,argon, helium, neon, methane, carbon monoxide (CO), hydrogen, oxygen,hydrogen chloride (HCl), hydrogen fluoride (HF), hydrogen bromide (HBr),nitrogen oxide (NOx), sulfur hexafluoride (SF₆), carbonyl sulfide (COS),volcanic gases, or any combination thereof.
 5. The method of claim 1,wherein said amorphous material has a lower specific gravity than othersaid green body materials.
 6. The method of claim 1, wherein said methodfurther comprises forming said green body around a template so as toencapsulate said template.
 7. The method of claim 6, wherein saidreactive sintering at least partially converts said template to atemplate comprising in-situ whiskers and at least one amorphous phase.8. The method of claim 1, said green body material comprising i. aluminaand/or at least one alumina precursor and a siliceous material in aratio to form whiskers in said glass-ceramic composite, and ii. the atleast one amorphous material containing at least one entrappedvaporizable material, and iii. the preformed whiskers, and iv. the atleast one whisker promoter, optionally in the absence of fluorine orfluorine compounds; sintering said green body under sintering conditionsto form in situ a glass-ceramic composite comprising at least onemullite whisker phase and at least one amorphous phase.
 9. The method ofclaim 8, wherein the median particle size, d_(a50), of said alumina oralumina precursor is from about 0.01 μm to about 100 μm, wherein d_(a50)is a median particle size where 50% of the particles of the distributionhave a smaller particle size.
 10. The method of claim 8, wherein themedian particle size, d_(s50), of said siliceous material is from about0.01 μm to about 100 μm, wherein d_(s50) is a median particle size where50% of the particles of the distribution have a smaller particle size.11. The method of claim 8, wherein the median particle size, d_(a50), ofsaid amorphous material containing at least one entrapped vaporizablematerial is from about 0.01 μm to about 100 μm, wherein d_(a50) is amedian particle size where 50% of the particles of the distribution havea smaller particle size.
 12. The method of claim 8, wherein said aluminaor alumina precursor has a particle size distribution, d_(as), fromabout 0.5 to about 15, wherein, d_(as)={(d_(a90)−d_(a10))/d_(a50)}wherein d_(a10) is a particle size wherein 10% of the particles have asmaller particle size, d_(a50) is a median particle size wherein 50% ofthe particles have a smaller particle size, and d_(a90) is a particlesize wherein 90% of the particle volume has a smaller particle size. 13.The method of claim 8, wherein said siliceous material has a particlesize distribution, d_(ss), from about 0.5 to about 15, wherein,d_(ss)={(d_(s90)−d_(s10))/d_(s50)} wherein d_(s10) is a particle sizewherein 10% of the particles have a smaller particle size, d_(s50) is amedian particle size wherein 50% of the particles have a smallerparticle size, and d_(s90) is a particle size wherein 90% of theparticle volume has a smaller particle size.
 14. The method of claim 1,wherein the fluxing agent is selected from the group consisting ofnepheline syenite, feldspar, synthetic clay, natural clay andcombinations thereof.
 15. The method of claim 6, wherein the templatecomprises a different composition than the green body material.